Push-Pull only works perpendicular to the face you're pushing/pulling.
If you need to intersect at an angle, usually what I do is draw the mating piece, get it into position, and then use the "intersect" features to make the proper geometry on for the female part of the joint (in this case, dados). Then hide the mating piece and adjust the geometry accordingly to get a dado-lookin' thing - intersect just draws lines so you'll probably have a few extra lines and faces to remove. Sometimes you also have to draw in the boxes created by the intersect to make it see the lines as a complete face (or else you get punched-through walls in your parts and can see inside where you probably don't want to).
Hope that helps - if it doesn't, a professional will be along shortly