Are you referring to the diamond shaped plate under the transmission?
When you say it won't go back on, are you having trouble getting one of the bolts in, or all of them?
If you can get most of the bolts in, I would take it to an alignment shop and have them reinstall the final bolt, and check the alignment, which can get thrown off by removing the brace.
Here is some more info on dealing with it:
http://986forum.com/forums/general-discussions/56230-rear-diagonal-brace-removal-question.html
http://986forum.com/forums/general-discussions/53796-can-not-get-transmission-cover-back.html