Unable to upload video to prezi using a Mac

Hi,

and help!

I cannot upload a video (between 1.8 and 5.2 MB in size depending on the file type) to my prezi, I have been trying for the past couple hours with no success. 

I am using a Mac and have tried this with Safari and Chrome. 

I have had a few different error messages, usually "file type not supported" but then it proceeds to tell me it supports a number of file types that include the one I'm using - I've tried MPG and WMV. 

Once an error message appeared that said, "id 2562 the reason is exception in callback" - but only had that once. 

Can someone please help? 

Many thanks
Karen