If you are importing into the existing arrangement (as opposed to a new arrangement) it will place the MIDI part at the left locator. So make sure you have the left locator at the song position where you want the part to be.
Does your library of parts have to exist as MIDI file parts? If not, one idea I had was that you can put your library of parts all on one arrangement. Then you can save the arrnagement as "library.arr". Then when you want to get some part from your library, you open your library arrangement and cut and paste parts from there into your song's arrange window. Remember to set the snap value to the resolution that you want the part to line up to.
What version of Cubase are you using? If you are using 5.0 on MS-Windows, note that you can also put your library of parts on the desktop or in a folder. Then you can drag and drop these parts from there into the arrange window.
