Yep, that's a bug we're aware of, and one of a few we're working on at the moment.
There is a workaround though... Right at the bottom-left of the forum view, there's a little drop-down box, which you can use to select the visual style/theme used to present the site (it probably says ASN at the moment). If you select either LegacyBlack or GrayFolio-BLACK (can't remember which one, or if both, off hand), the Product Options menu will work so you can edit your ad; of course once that's done you can pick whichever flavour you prefer the forum in.
I'll be taking my hammer to that one in the near future... I promise!
Cheers,
Rob.