It's easier than you think. In the project timeline, position your cursor in the the spot where you want the MIDI file to begin playing. Select File > Import > Import MIDI File. A prompt will appear and click on the NO button. Now browse your hard drive to find the file you want to import, select the file and click on the Open button. This tells Cubase which file you want to use and where to insert it. Naturally, the MIDI File will play back at the tempo you set for your project.