Black stuff under freezer & freezer not cooling properly

My freezer part of fridge/freezer has been having trouble achieve low temps for about 6 months now. It wasn't much of a problem, the meat seemed frozen but the ice cream was too soft. The problem has gotten worse. Today I pulled the fridge out to have a look for the first time and I found that there is a black residue that has leaked under the freezer. It is dried now. I am wondering if this is worth repairing or if I should get a new fridge. Thanks. Click here for photos of problem.