is there a songpicker in realband like the one in band in a box?

Nope...you would just use "File, Open" to find the song you want to load. And remember that RealBand opens SEQ files (the RealBand and PowerTracks format), BIAB files, MIDI files, Audio Files, MP3 files, etc.

Just in case you mean "Style Picker"... That does exist in RealBand. To access it, click on the loaded style's name (located under the title of the song).