Checkout source from Subversion

Go to File / New / Other and select Maven / Checkout Maven Projects from SCM:

Select the svn type and type the url https://openkm.svn.sourceforge.net/svnroot/openkm/branches/4.0/openkm to refer version 4.0:

Be patient, first time you downloading OpenKM from svn, you're downloading all libraries to your maven repository. It could need some hours depending your Internet connection.

After OpenKM it'll be downloaded, automatically it'll be compiled.