Official read-only access to Mercurial repository is provided through
hg clone http://hg.code.sf.net/p/roundup/code roundup
Read/write access requires SSH password or SSH key authorization (see SourceForge.net docs for details)
hg clone ssh://USERNAME@hg.code.sf.net/p/roundup/code roundup
You also need to be added as a Roundup developer for write access - ask for it on the roundup-devel list.