There are three methods:
Three Team Project sets are available here on the web site for download:
You can also get them from the IMP SVN repository. The "root" project set file
is in the project org.eclipse.imp.update, and is named features.psf.
It lists all of the various IMP feature projects (for the runtime, the meta-tooling,
the Java-hosted language support, program analysis, program analysis UI, and the LPG IDE).
Each of the feature projects in turn has a Team Project Set plugins.psf that
lists all of the plugin projects belonging to that feature. The following diagram
illustrates the organization:

Using the IMP RelEng Tools
First, you need to install the RelEng tools from the
IMP RelEng update site.
Once you have that installed, proceed to the top-level "Release Engineering" menu, and select
Retrieve Update Site Project. Select "IMP" in the ensuing dialog, and you can also check
"Also retrieve feature projects" and "Also retrieve plugin projects"
in order to get all of the IMP source in one shot:

If you don't check the "Also retrieve" boxes (or if for some reason it fails to retrieve one
or more projects), you can then import the Team Project sets in the normal Eclipse manner,
via either File -> Import, or by right-clicking on the
project set file in the Package Explorer and invoking Import Project Set.
Please note: You'll need to have already installed an SVN accessor,
such as Subclipse.
Checking out from the IMP SVN repository
Simply go to the SVN Repository Exploring perspective, and create a
new repository location for IMP via New -> Repository Location...
pointing to the IMP repository at http://dev.eclipse.org/svnroot/technology/org.eclipse.imp: