Compression greyed out

[jason]

I'm new to Entourage and I'm trying to compress a file. In the
attachements pane after clicking the flippy triangle and expanding the
attachement box I get the option bar which says "Encode for any
computer/No Compression" When I click that bar to change the
compresison option to Stuffit, the Compression options are greyed out.
Anyone know what I'm doing wrong?

Thanks.