Before anything else, you can't "upload" a video from your PC into an email in Prospect Reach.

But instead, you can upload the video to any video hosting sites such as Youtube, and Vimeo.

Once you've uploaded your video, you can then grab its embed code and paste it on your emails in Source code view.

Note: Make sure that the video is set to Public.

Here's a video showing how it's done with Youtube and Vimeo videos: