you could use ksDSSP, which is an OpenSource implementation of DSSP.
Requires some compiling though and I am not sure if OSX is supported.
You also need to compile the PDB record I/O libraries.
It runs fine on OS X, but I think I had to run 'ranlib' on the PDB library to get it to compile properly. However, I've seen at least one structure (1ywf) for which it produces incorrect results; this may not matter if you just want to make figures, but it's problematic for any other uses.
I was able to download the DSSP source code from CMBI without signing anything - but it's still proprietary and only free to academics. (And I had no luck compiling it on a Mac, unfortunately.)
-Nat
