As of tkoutline version 0.93, some disabled incremental search functionality exists in tkoutline. It performs incremental search within the current active outline. In order to enable this functionality, place the following in your startup script (Edit->Edit startup script) and restart tkoutline:
event add <<SearchForward>> <Alt-s>
Or you can open the console (<F2>) and type the above command to activate it without restarting tkoutline.
This will bind <Alt-s> to start an incremental search. It doesn't even have a "find again" feature and I really don't recommend using the feature until it is completely implemented in the future. Once you hit <Alt-s>, you enter search mode and anything you type is incrementally searched for. Backspace will backup on character and Esc will leave search mode. Be warned that nothing visually indicates when you are in search mode or not.
There is no way to search external outlines. Tkoutlines files aren't very readable, but they are text based. Headlines are on different lines from each other, so it is possible to use external line-based searching programs to do searches (I use grep). It is far from ideal and it does work.
A user adds (30Jan06): I tried this (tkoutline v0.93, under Windows XP) but it didn't work. I hope you'll add it soon. Internal search seems like a basic feature for any database program.
Brian Theado - Whoops, the above description had a typo. It said to put the line in your preferences file and it should have said put it in your startup script. I have fixed the description. Let me know if it still doesn't work.