Click on your project name in the navigator. Choose "Build Phases" from the list header in the right hand window. You should see a bar named "Compile Sources"; expand it and you should see a list of the source files in the project and a column labeled "Compiler Flags". For every file in the Mixpanel SDK, double-click in this column and add the following line:
-fno-objc-arc
This will signal the compiler to not use Automatic Reference Counting when compiling these files. If you ever need to do the reverse (compile ARC code in a non-ARC project) do the same thing but add this line instead:
-fobj-arc
I believe this may be a duplicate of this question.