If its a fresh ISBoxer, its possibly your in game key binds.

Since its not working at all, Id troubleshoot it. Reset everything to default, including WoW Keybinds. Also double check your JAMBA and ISBoxer AddOns are properly checked. Use the default hotkeys for ISBoxer. Put your mount skill in an action bar, and map it, press the mount to mount them all, and then fly up a little, and press ALT+F to ensure the follow command is not interrupted.

There should be no reason why that shouldnt work. If it doesnt.. then something still isnt configured right.

Did you double check your Character Set profile so that it includes all toons, and the proper keymaps? including the jump?