Just picked this up from the forum's "feedback" forum: Strip the Search for: field and click the button. This will bring you to the new window, where you can select More options (toggles to Fewer options as shown in this shot) and then narrow your search parameters.
Unfortunately, scrolling through the list is problematic, since it's tough keeping track of where you are because all sub-forum categories are listed for each product forum (and there are a ton of them!). Now, you can enter the actual search string in this Search for: field.
Hopefully, Adobe will polish this up soon.
Using Google to search the Adobe forums is still faster (and more efficient?). Add "site:http://forums.adobe.com/community/robohelp?view=overview" (without quotes) after your search string. This will only find posts/pages within the RoboHelp forum And it's much, much, much faster than the Forum's search tool.