Hi Marie,
While in edit mode, go to an individual's page (either individual in the marriage). Click on the Edit icon to eneble editing of the person. There you can select the "Citations" tag. When you click on the down arrow in one of the citation positions you will see something like "Marriage with xxxxxxxxxxxx". Select this, complete the source detaild and when you save the result it will show up for both people at the end of their notes (if any).
May not be explaining this really well as TP is down at the moment - which has given me time to got through all the forums.
Regards,
Ian Marr