I'm looking through the questions I've asked and chosing the mho, and I clicked yes to the pop up window that said "do not show again on this website" when confirming the mho (something like that). Now nothing happens when I click on the mho star, so I can't chose the most helpful opinion.
Which page does this happen on? Are you on the individual question page and trying to select MHO there? When you select a MHO, you shouldn't get a notification that says "do not show again on this website" so can you tell me what you clicked to give that pop up message? When you click the star to select a post as Most Helpful, the pop up message should have said "Are you sure? You cannot change it!" and then you select yes or no.