Do you want to continue to use your Makefiles to build, or are you trying to configure things to use SlickEdit's build system?
If you simply want to configure a project to build using your makefiles, I would start with the C/C++ project wizard and select to build using a user written makefile. You may have to point it to the location of your makefile. This should get you started with the minimal amount of effort.
If you want to use SlickEdit's integrated build system, then again, I'd recommend starting with a generic C/C++ project, then configure the compile and link comands to use clang instead of cc.