@avkiev: There are 2 ways of doing it (assuming you don't want to test it by pressing search on the remote):
1. Use a web browser and navigate to
http://<your.wdtv.ip>/umsp/umsp-test.php. Navigate to your plugin and if search is implemented you'll see a search box on your page and you can test the search functionality really easily
2. You can implement an item that when called would pass a special argument to your UMSP script (something like doSearch=xxx). Then, on the server side you can test for this and call your search function. But beware - you can't ask for user text in this step, since the OSD will not display a keyboard.
Hope this helps.