I've been attempting to make an Undead warlock, and have purchased VRGTR, but don't have the option to add it in the character creator. I also can't create a homebrew subclass based on it. 

Any assistance on this greatly appreciated. 

Go here, and check two things: under account entitlements, does it say they're in sync? If not, there should be a button or a link to click. Also, double check under 'Current Product Licenses' that you did purchase VRGTR, and as a sourcebook and not a compendium (which would have been $19.99 instead of $29.99). If both of those are true, then depending on how long it's been since you purchased VRGTR (sometimes it just takes an hour or two for things to sync at initial purchase) then you might need to open up a ticket with support.

Thanks for actually directing me here, it made me realise I'd never actually bought Van Richten's, was just in a campaign sharing it. Have now bought and all good! 

You're welcome!