I have found the best results for connecting the Mix Pre-D to a DSLR (in my case the D800) was to go from the "Tape Out" jack (not the "Stereo Unbalanced Mic Output" which has a much lower level signal) with a 1/8" male to 1/8" male cable to the Mic In on the camera, while lowering the Mic level on the camera down to "2" or even "1." You get a hotter signal out of the Mix Pre-D which allows you to almost eliminate hiss (which only starts becoming audible when you raise camera Mic Level above "3" or so) and still capture a high fidelity signal. Not sure if all this holds true with the 7D, but worth a try if you have ability to control input gain.
The "Tape Out" jack is found directly underneath the "Unbalanced Output" in the Mix Pre-D. It has the added advantage of accommodating a common, inexpensive type of connector, whereas the "Unbalanced Output" requires a TA3 to 1/8" cable which is harder to find and not so inexpensive.
BTW, the Sound Devices manual for the Mix Pre-D may state that the "Stereo Unbalanced Mic Output" was designed to interface with DSLRs, but that may or may not always be the only--or the best--method.