Can someone tell me how to mount a direct drive motor to a 3DX?

cut off the extra foam in the way of the motor and use two small zip-ties to secure the motor to the mounting stick. Now why would you want to use a direct drive motor on a 3dx? With a flat plane wing the only thing that makes it fly is thrust and a gearbox and big prop are the best way to get it.

Maybe a fat high torque motor?

I have a GWS-400 direct laying around so I thought I'd use it. Would that be enough power to hover a 3DX that weighs about 10 oz.
And could the motor hold up to a Li-Poly 3sp 1200 mAh with out frying.

You will probaby kill your batteries and you wont like the performance of a dd 3dx. Buy the parts to build a geared gws 400 or better yet buy a 350c(D gear) from aeromicro.com $17.50 along with 12x6 prop and youll be set

Would the 350-c D hold up to the battery?

Is the pack an Etech? Well anyway a 300 c (D) will probably be fine on your pack on an 11x7 propm but the 12X6 will probably hurt it if you fly too much full throttle. It all depends on the amps you are pulling and the C-rating of your cells. I havnt tried 1200s but have used TP 1320s 2s, Kokam 1500s 2s, and Kokam 1050s 2s times 2 and they all worked fine, I like the 1050s the best because I use two 2s packs with a homemade y-connector and have one pack on each side of the fuselage and it really helps balance the plane