On the support page they say they have a FAQ, but I think they forgot to add it to their site.

If you're not a registered user (& thus ask their support) then perhaps play around with small video files to get a hang of it. Select a small video file & choose to convert to the format you desire.

My guess is you press the Add video button, then press the convert button after selecting the file in the list. But check the options button also.

My windows system runs very slowly & would take me a few hours to try it out for you. But I will if you desire.