I just did the quest to retrieve the plans for the Dwemer Crossbow and she told me I could make it. Well, I went to the forge, went to "Dwarven"...and..it's not there? What do I do?

do u have the perk that allows dwarven smithing? that might be your problem but idk cuz i havent got the dwarven bow either

Yep. ^^

Did you use the forge at Fort Dawnguard?

This. You can only craft them at the forge in Fort Dawnguard.

The forge inside fort dawnguard were gunmer is at. you cant make bolts or crossbows anywhere else.

I have the same problem, i can smith all other dwarven sutff except the crossbow. This is at the dawnguard forge.