I'm not sure if I completely understand, but if you had the source under Marriage Fact and wanted it under Marriage License, I would:
1) Add a Marriage License Fact, using the "+" symbol at the top right of the Person Page. Fill in the date, place, and description with whatever is appropriate.
2) Go to your Marriage Fact and select the CITATION. Then click on the "copy" button in the button bar (looks like 2 pieces of paper overlapping each other.)
3) Go to the Marriage License Fact you created in Step 1 above. Click on the itty-bitty little arrow next to "new" and choose "Paste Link to Source Citation."
4) Go back to the Marriage Fact and select the citation that you copied and choose to "unlink" that citation. That's the "broken chain-link" in the button bar.