If your professor disabled the ability to download the lectures, here’s the way to get around it. First, here’s the snippet you will need; then read below it for instructions.
javascript:__doPostBack('__Page','DownloadPresentation:1299fb37-f10a-31be-97fe-0224842e6cdd')
On the lectures list, hover over the link to the lecture you want and copy the peid from the URL. Put it after “DownloadPresentation” in the above JavaScript snippet, and add dashes exactly where that peid has them – you should have an extra “1d” on the end which you should just delete. Then run that javascript on the lecture list, and the download dialog will pop up.