The "Click" functionality is essentially saying, press all of these things at once.

It is the same thing as making a macro with every spell in one line.
Normally this will work up until you have a 2nd spell which also incurs a global cooldown.

So:

/Use Icon of the Silver Crescent
/Cast Elemental Mastery
/Cast Lightning Bolt
/Cast Flame Shock
/Cast Lava Burst

This functions up until Lightning Bolt.
Anything past that point fails, because LB incurs a GCD, which FS and LvB also incur.

By having the Clicks call castsequences we get around this.

A castsequence will do one thing and then the next.
With two castsequences running at once, the first to be called runs first.
The second one tries to run, and like Flame Shock or Lava Burst fails to do anything.

However, unlike just Clicking abilities or running my above macro, they are a sequence.
Part of the sequence are comma's which function as "Nulls", which means nothing happens.

So we have click one activating the Lightning Bolt.
And nothing else happening.

Click two activates "Null" on the first sequence, which means no GCD is incurred.
This allows the second castsequence to do something. So Flame Shock will fire off.
The Lava Burst fails because Flame Shock used the GCD it needs.

The 3rd click gets a null on both the LB and FS castsequences, which allows LvB to fire off in the 3rd sequence.





Generally when you're building these sequences, you'd like to follow these steps.

Go to a target dummy, and figure out your DPS sequence.
You'll generally have several abilities you wish to use.
In your case it was Blood Strike, Scourge Strike, Obliterate and Death Coil.

I don't know exactly how a Death Knight works, having not played one since the Beta.
I do know abilities use Runic Power, which you need to have enough of to do them.
I also know an ability generally costs a rune type, and you have two of each.
So you could not do a Frost power, another Frost power and a third Frost power, until a Frost Rune refreshed.
That's about the limit of my knowledge of DK powers.
I don't know which of these powers you want incur which rune types or how much runic power they require.

To build a strong DK rotation, you'd probably want:
a) a Blood Ability.
b) a Frost Ability.
c) an Unholy Ability.
d) a 2nd Blood Ability.
e) a 2nd Frost Ability.
f) a 2nd Unholy Ability.
g) a Runic Power dump or an attack which doesn't use any rune type.

You then need to know how fast you mash your click sequence key.
Get a stopwatch or watch the clock for 15 seconds.
While watching, spam something like P or R; anything where several in a row are easy to read ( PPPP has several down strokes which are easier to count then say XXXX ).
Spam away, and then after 15 seconds count how many you typed.
Divide that by 15, and you have your clicks per second, or thereabouts.

Now you want to take your first ability.
Figure out the recast delay or how long until you'd like to cast it again.
Something like Chimera Shot for a hunter would be nice to cast often.
Something like Flame Shock for a shaman would be better to cast just as it expires.
Figure out how long after you cast it the first time, until you'd like to cast it a second time.
I personally click at 2 clicks per second.
If I'd like to do a Flame Shock every 18 seconds (12 seconds duration, but +6 seconds duration glyphed...)
In the first second I click the spell and a null, so 18 - 1 = 17 seconds of delay, plus my null.
So my first sequence is Flame Shock plus one comma plus [ 17 (my desired delay) x2 (my click speed) ] = 34 comma's.
So I go with /Castsequence reset=combat Flame Shock,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,

That's a total of 35 comma's for my click speed.
One comma for the second click in the second that the spell fires off.
34 additional clicks for the 17 seconds of delay, given my 2 clicks per second.

Now that you have your first ability done, you'll want to choose a 2nd.
Once that's done, choose a 3rd.
Repeat for as many spells/abilities as you wish to include in your sequence.
Ideally put an ability you can spam at the end for your fall through.
The "Fall Through" will trigger when all the other sequences are on a "null".
This way, you'll always be doing something if your toon can (is not waiting for a GCD).