How do I use the debugger in xcode?

Most code I've had to write before has just required simple debugging and I've never had to make use of a debugger so this is all new to me. I've got a piece of code that has a bug in it somewhere that causes a sigsev error. I worked out that I needed to use the "Enable Guard Malloc" option and the debugger has kicked up an error of "EXC_BAD_ACCESS" in what appears to be asm GMmalloc. Armed with this information, how do I find out which line of code (all C based) is causing the error? Unfortunately I haven't reached the level of "The One" yet to be able to read assembly code like a book!!

Edit: I think the bug may be being caused inside the fftw3 fink library the program causes, even more stuff that I don't really understand!!