The makefile only creates the executable. As the manual explains,
it's up to you to put this executable somewhere on your path. Or you
can invoke with a full path. For example if you are in the source
directory right after the make, you can say
./sketch -h
and the help message should be printed.
I'll also add that the makefile is meant for developers. It might
work for you, and it might not. If you didn't see any error messages
when you ran it, you're probably fine. It has never been tried on a
lion machine.
The simplest way to build the executable (after installing a compiler
like XCode's) is to say:
gcc *.c -o sketch
This is also explained in the manual.