Just click on exec to get a list of all mp3s in the current selected directory. The Results will show up in the list, which you can now browse comfortable. Click a column-title to change the sorting!
To apply a filter which files should be displayed chnage to the filter notebook page by - you guessed it- clicking on "Filter" at the left. Now just enter some text in the fields.
If you select "exact match" only files will be found which have exactly this text, without it YAMT searches also for substring. So eg. if you enter "age" somewhere (and of course check the button right to the entry) it will find "Rage", "Rage against.." and "FooRageBar".
To work on single or multiple files mark them with the left mouse button and then right click so that a popup menu appears.