Hey Thomas,
When you come up against a JS error or conflict the only person who can really resolve it with a code fix is the original plugin developer – For a start they know their plugin, and secondly their fix will continue to exist through future versions. If you really knew what you were doing you could probably spend some time trying to track down the problem and fix it, but it would be overwritten with the original once updated.
Where Andy has pointed out that ‘main_menu.dd’ seems to be causing problems, we are suggesting you find out what plugin (or possibly your WP theme) this is coming from and deal with it. The majority of times this just involves removing the offending plugin or switching to another theme, but if you find out this is coming from something you can’t live without you would need to contact the plugin / theme developer and ask them if they can resolve it.
A basic way of going about this would be to disable ALL WP plugins, see if you can then use the ‘Add Topic’ button. If you can, enable plugins one by one until you find the one that stops you from using the button.
Unfortunately there are lots of plugins and themes out there that don’t conform to the WP API which cause knock on effects with plugins that do – Such as Simple:Press.