How to import home movies from movies to iMovie?

Hi

I just purchased my first Mac. It is a IMAC. I backed up all my movies from my PC onto a external harddrive. I was able to successfully tansfer all the documents and pictures from the harddrive to my MAC. The place where I am stuck is my home movies. I moved all my videos to movie but when I open IMOVIE and click on the file they are grey and you can select them. Do I need to download a program to convert them?

I appreaciate any help that you can provide me.

Thank you