If the list of sources you see on the source screen (Plan Specifications=>Work with Sources/Inv Accounts), does not include all the sources used by the plan, click on the Add Default Sources link. This will add any missing sources.
If you are attempting to add a default source and it does not appear when clicking the link, check your plan specifications; e.g. if if the plan has a safe harbor match and you don't see that source, even after clicking the Add Default Sources link, check the Contribution section of Plan Specifications to verify that the plan is coded as safe harbor match.
Note that if there's a funded source that is not current, e.g. the plan used to be safe harbor nonelective but has switched to safe harbor match, you may have to temporarily code the plan as safe harbor nonelective and click the Add Default Sources link; that will add the NONELECSH source. Then switch back to safe harbor match.
Try to avoid add sources manually; manually added sources do not allow contributions. In rare circumstances you may have to do this, e.g. an old Money Purchase source.
Below are the SourceIDs used by the Compliance Module: