Works fine for me, I'm also on 3.5.1...
When you say '... Extra > Add-ons ...', I'm assuming you mean '... Tools > Add-ons ...'?
The version that you download from the Addons site isn't explicitly updated to work out of the box with Firefox 3.5.*, but you can easily fix yours...
Download it to your desktop then open it (I use 7zip, which treats .xpi files like zip files, so it opens like a folder), then Edit the install.rdf file, and in the Firefox section, change the '3.0b3pre' to '3.5.*', then save the file back into the .xpi file, and close it.
Now, install this updated extension, and it will work out of the box on 3.5.* versions of FFox...
If you have trouble doing the above, I can email you my copy of the extension if you like...