Dear Rafael,
usually to preload a Multi Selection Field, it's used a Selector Unit with a transport link that sends the data you need to preselect the Multi Selection Field in the Entry unit. So the coupling parameter to use will be the " Selection Field Name Preselection."
If you have already done what I've described, the error probably is dued to the design of the page. In this case, could you attach here a screenshot of your modeled page?
Regards,
Mario