The CVS repository is at SourceForge. It can also be interactively browsed at:
http://iup.cvs.sourceforge.net/iup/
To checkout use the module name "iup" and the CVSROOT:
:pserver:anonymous@iup.cvs.sourceforge.net:/cvsroot/iup
To checkout the IUP 2.x source code use the module name "iup2".