The Mahout source code resides in the Apache Subversion (SVN) repository. The command-line SVN client can be obtained here. The TortoiseSVN GUI client for Windows can be obtained here. There are also SVN plugins available for both Eclipse and IntelliJ IDEA.
Web Access (read-only)
The source code can be browsed via the Web at http://svn.apache.org/viewvc/mahout/. No SVN client software is required.
Anonymous Access (read-only)
The SVN URL for anonymous users is http://svn.apache.org/repos/asf/mahout/trunk. Instructions for anonymous SVN access are here.
Committer Access (read-write)
All bugs, improvements, patches, etc. should be logged in JIRA.