Are you saying that you can't set up a signature or that after you set it up, you can't access it.

To set it up with 2007, I do this:
Make sure you are in a mail section- the options don't work in other sections.
Click tools in the top menu.
Click options
Click mail format
Click Signatures

The window that pops up from that should let you create a signature.

I can't find another option to get that window. If no one has a better idea, you might consider reloading Outlook.