Where do I find my mystery gift?

IDK but the places I would look (and that you may have already) are sim inventory, household inventory, Store>All>Custom, Downloads Dashboard and maybe even the mailbox.

Check your purchase History: https://store.thesims3.com/purchaseHistory.html

I also can not find my mystery bag gift. I have checked my purchase history but I do not see it anywhere.

Did you get any before or is this your first ?

It can take a moment until it shows up but if it doesn't, you need to contact customer support

They can be found here: https://help.ea.com/contact-us

Choose a category, a game and a platform for the contact options to show up.

If live chat is gray, change your country to US and make sure you're on US Business hours. It might also gray because they're very busy. Try later.

See this post if you have trouble with the site: http://sims3.crinrict.com/en/2012/10/how-to-contact-ea.html

Beware that there's no longer an Email option