The OLAT source code is splitted in two java Eclipse projects: the olatcore project (cvs module: "olatcore") and the LMS (learning management system) project (cvs module: "olat3"). The olatcore project is the framework upon which the LMS is based on. Basically, one only needs the olat3 for LMS development, since per default the olatcore lib is provided as jar in the olat3/webapp/WEB-INF/lib .
For debugging purposes, there is the possibility to checkout the olatcore and link the olatcore source to the olat3 project build path (Eclipse: olat3 project Properties/Java Build Path/Source/Link Source). It is advisable that the olatcore should not be changed during the LMS developing process.
The OLAT project gives you read-only access to its sources. If you encounter bugs or have some interesting hints or questions about parts of the source - do not hesitate to write it to the developers list.
You have to open the CVS Repository Exploring Perspective, which you find via Window | Open Perspective | Other..
create a new repository location: right click to create a new repository location

fill in the needed fields:
host: cvs.olat.org
repository path: /usr/local/cvs
user: anonymous
password: anonymous
connection type: pserver
use default port
validate connectoin on finish
save password
click on finish, this validates if you can connect... you should be connected to internet to have this working.
After successful creation of this CVS repository location you can browse your repository. Just go straight ahead to the HEAD | olat3 right click on it and check out this copies the whole project from the repository server to your Eclipse workspace. This may take a while...

As already mentioned, the LMS java project already contains the olatcore jar (olat_core-1.0-SNAPSHOT.jar).
In case you also need the olatcore source:
checkout the olatcore module from HEAD.
Define a M2_REPO classpath variable for your maven repository.
Define your maven repository to Eclipse by running the maven command:
$ cd workspace_dir (go to your Eclipse workspace directory)
$ mvn -Declipse.workspace=. eclipse:add-maven-repo
This command creates an Eclipse classpath variable M2_REPO that points to your local maven repository.
IF THIS DOESN'T WORK: add M2_REPO class path variable manually into Eclipse (olatcore project Properties | Java Build Path | Libraries | Add Variable | Configure Variables… | New)
cd olat3
mvn eclipse:eclipse
mvn install (install copies all jars and resources to the target folder). If you run out of memory while running this command set MAVEN_OPTS="-Xmx1024m" in front of the mvn command.
In Eclipse: refresh olat3 and olatcore projects.
If you want to work/debug in both projects (olatcore and olat3), you must change the default olat3 project settings like described in Section 1.6, “Optional: Use olatcore source code instead of precompiled jar-file ”