First click on the sun/moon sphere at the top of the screen then try crtl + alt in the working window. Otherwise if you have your camera facing with a Y rotation of 0 degrees then if the sun is at the top in the sun/moon sphere the sun should be directly in front of the camera. Go into the sky lab, change the preview window to "render in scene" and lower the sun until it is around 5 degrees altitude (azimuth 000). You should see the sun, then just place where you want it using altitude and azimuth.