Help! I'm not real good with my 1-year old MacBook Pro. [View all]

All of a sudden, I've been getting these messages on YouTube that says "blocked plug-in", and it won't let me play any YouTube videos. I CAN connect to them on DU, but only if I click on the video screen embedded in the post. If I want a full-size screen shot of the video, and click on the YouTube icon at the bottom of the video to go to YouTube to watch it, I get the "blocked plug-in" message again.

I've tried to use the help icon, but it doesn't get me anywhere that looks like I know what to do.

I haven't done anything that I know of to begin blocking "plug-ins". My computer has been acting really weird with this and a few other minor changes that sort of just happened about a week ago. I don't remember downloading any updates in the past week.

Can someone help me stop this "blocked plug-in" message?

Thanks in advance.