This is also in the forums.
Multi Select boxes are defined in Marketo as
Value 1;Value 2; Value 3
so you need to use the CONTAINS or actually list out the combinations you want. You must do this as well in the Form. This is one reason I do not recommend Multi Select use in Marketo.
This also means you need to be careful with list uploads because CSV and semi-colons could get confused by some people. But it should work if you do one column and then use semi-colons inside each "cell".
if you divide the CSV into separate comma separated columns, you will have problems. I'd run some tests to figure this out for yourself.