Ahhh, I know what the problem might be - I have it set that a new tab opens when I use the addressbar of the current tab (maybe a TabMixPlus option, cant remember offhand). That would screw up the java script I reckon (like the way you cant open java scripts in a new tab).
I'll stick with the bookmarklet (actually I've added it to my homepage - that way I can use the homepage icon & dont need another toolbar open)
I have mine configured the same way, except I am using Tabkit.
You may still be able to use it with a keyword from the addressbar. Try using alt+enter to make it work in the current tab instead of using just enter, which causes it to open in a new tab. (or just click the little green arrow on the far right of the addressbar after typing in the keyword, instead of hitting enter)
This is far better than a dedicated button, since a button can only do a single thing, while you can use keywords for all of your frequently used bookmarks, without cluttering up your toolbars.