First things first: read the Installation Manual for 2.4. There's more to installing WeBWorK than just downloading a tarball. But if you just can't wait, or want to get a look at the code, read on...
WeBWorK can be downloaded from SourceForge or checked out of CVS. We recommend that you install WeBWorK from CVS, since you can get more up-to-date code that may contain bug fixes that haven't been released yet. It also makes updates easier.
The current version of WeBWorK is 2.4.1. The current version of the PG Language is 2.4.0.
Download WeBWorK and PG from SourceForge
Download WeBWorK and PG using CVS
You most likely want the bugfixes branch for the current version. To grab that from CVS, type the following:
cvs -d :pserver:cvs.webwork.rochester.edu:/webwork/cvs/system co -r rel-2-4-dev webwork2 cvs -d :pserver:cvs.webwork.rochester.edu:/webwork/cvs/system co -r rel-2-4-dev pg
See the Installation Manual for 2.4 for details.