I thought this should be a basic and easily available thing to find but I can't find my XP anywhere. How much I have, what I can do with it, etc. Not in my account, home screen, checkout screen, not mentioned in the last 6 pages of the forums, not in my wallet screen......

Thanks

You can see it here. Should you get more than the required number (5K, 15K, 30K), you can also redeem the respective game to your account, or get a code for that game if you already have it.

The link is on front page (see attachment). Unlike Steam, the XP has nothing to do with you account ranking and is only applicable during the sale and disappear after it's over. Badges only give you XP during the sale. Only real use for the XP is (as JMich already pointed out) that you get free games (or gift code(s) if you already own game(s) in question) at certain XP levels. In case you are wondering, you can't get enough XP for "free" game getting badges and doing check-in's (Max is 4600 I think unless there's more events). You will have to spend some to get it. You get around 1000 xp per 1€ spend.