I have a bunch of songs in .mid format that I did in Cubase windows, I can t load these in El Capitan Mac. Is there a way that I can still access these tracks in iMac or are these one available in the windows format 
.mid files are MIDI files and are not application or platform specific. If you double-click on a .mid file, it should open in QickTime Player, by default. You may check that the default setting hasn’t been changed by right/Control clicking on the file and select “Get Info”. Make sure that “QuickTime Player (default)” is selected in the file info. (You can select Cubase as the default if you wish.)
The easiest way to open a .mid file in Cubase is to simply drag and drop it on to the Project window. If this doesn’t work, the file is probably corrupted, or isn’t a MIDI file at all. You say you did them in Cubase for Windows. Did you save them, or did you export them? There’s a huge difference!