How do I download a short youtube lecture for a powerpoint presentation on my macbook pro?

Google does not normally allow downloading of their copyright videos unless there is a download link in the video. Read Google's YouTube TOU. So maybe your question is better asked of YouTube or Google.

There are third party apps that can download YouTube video but check the legallity before doing so.

I use media center mainly for airplay to AppleTV on older Macs that don't offer it as standard… but it works well for downloads too.