Indeed, they can take a little while to show up, I had one that lingered for an hour myself.

On the bright side though, unlike the level 90 boosts from WoD, the level 100 boosts are Battle.net account-wide. In other words, you can make that level 100 on any WoW account associated with your Battle.net account, not just the account you pre-purchased Legion for (provided it does have WoD on it so you can actually play to level 100, of course). Yay for more flexibility!