The jBPM Eclipse tooling comes with the Guvnor perspective which you can use to connect to your Guvnor instance, but AFAIK it only allows to upload assets to the repo and not download (please check). Another option would be to use the Guvnor REST API. For example if you have an asset named myAsset in the defaultPackage package, you can do:
http://localhost:8080/drools-guvnor/rest/packages/defaultPackage/assets/myAsset/source to get to its sources.
Hope this helps.