if you want to find something in a specific log, you can just press CTRL+F in the history viewer and this will open the find dialog. If you want to search ALL your history files for something you could always use windows find files/folders feature, just browse to the history directory and Search for Files Containing Text: whatever you want to search.
A built in universal search (all history files) in AIM+ would be easier so I agree this feature should eventually be added, but until then I hope the above information helps some people.
If your operating system supports it, I know 2000 and XP do, you can just search for text in the AIM+ history folder, that's what I am doing at least until this feature is added.