How to isolate a refrigerator?

Depending on the age of the fridge, you may have some compressor mounts that need replaced and that could reduced the transmission of sound. Also try foam insulation in the ceiling below the fridge if accessable.
If not accessable try drilling a hole in the floor in the middle of the fridge footprint area and blow in insulation either foam or dry type. That patch for the hold won't be visible unless the fridge is out.

Maril555 that A/C unit is most likely sitting on a concrete covered foam platform that HVAC companies use when doing installs. They dig down about 2-3" and through some sand down to work this foam platform down into it and level it. There really should be no transmission of vibration from mthe unit to the house except through the refridgerant lines. It really is de-coupled from the house.
As far as the refidgerator. I would try to have a freind tip the "empty" appliance forward and use one of those "grabbers" you see in some hardware stores for people to reach items on upper shelves, and place the pads under each rear corner. Then lift the front and slide the pads under each front corner. Just a suggestion.

Moving the A/C unit could solve the problem but the longer the lines the less efficient the A/C is. But a small trade off for the solution you seek. If you do have it mived have a extra thick concrete platform poured. That will help in vibration. One thing to answer your question is, yes the a/c unit needs to be level, and isolating the tranfer lines from the wall pass thru will help too.