Office 2008 - Alt Text for Pics

Our gov't org needs to make compliant docs. This means adding "alternate
text" to graphics. Office for Windows could always do this via the
"Format"
dialog box. Office for Mac never - the box was there but it was
greyed-out.
Thought I read someplace that you could in 2008. Anybody familiar with
this...? I don't even see the greyed-out box in the "Format" area anymore.