Encode Video Files not working for avi files

Hi,

I've been trying to convert some .avi files to apple format using Lion's native 'Encode video files' function however keep getting a 'avconvert: source file not found for source file://fileparthhere avconvert: failed to create an export session. Check setup

I can't work out why as this has worked before in Lion and works fine for other video formats. The only thing I've done is remove Final Cut Express HD and can't add it again as I don't own it anymore

Suggestions? Is there a encoder to make this work or should it work anyway?

Thanks