What formual do I use to return a value from a multi-picklist field

I created a Custom Object and created a report to show the content. I need a field from the Accounts object, which is a multi-picklist field. Building the report from the custom object doesn't allow me to select that field from the Accounts object.

So, I thought I could create a new field on the Custom Object and build a formula that would copy the value from the Accounts Object. How would I complete this formula?

Regards, Dave