This may be old news to everyone, but after having to deal with this recently I figured I'd drop in and mention it here, what with the Express version being free n' all.
Hitfilm is a non-linear editor, and apparently most NLEs choke on so-called "delivery" formats like WMV, MOV, or the like. My own experience is that WMV doesn't work so good, and AVI (the only non-.webm format MovieStorm exports) is a total coin flip. They'll import them, but once you actually try to play them back from within the editor or do any actual editing, you'll have all sorts of problems like lag, stuttering, or (in my case) completely mismatched audio.
The solution is to use an "editing" format (I use .m2ts), as spelled out here;
and, as an aside, I use the excellent V2V (aka "Video2Video") program; it converts pretty much any video format to any other video format, and ditto for audio (and it doesn't trip my virus scanner like Any Video Converter does; apparently the newer versions have spyware). It also works in batches, which was damn handy when I had to convert an entire directory of .ogg files to .wav format (...did I mention HitFilm doesn't do .ogg?).