Just run 'mplayer /home/user/MyDocs/.videos/Discovery.avi' without './' part. It would be needed if you had mplayer binary in the current directory and wanted to run it, that was the case with older non-packaged versions. Now mplayer binary is installed in the system and can be run from anywhere.

As for Discovery.avi video, it is not the best file to play by mplayer right now. Mplayer is optimized for video resolutions that are exactly twice smaller than the screen size (320x240 and 400x240 for example). For such resolutions it uses hardware pixel doubling and works fast. Discovery video is smaller than that, so it needs to be scaled and scaling takes a lot of cpu resources. If you want to get the best performance for Discovery video, you can run mplayer as 'mplayer -vo x11 /home/user/MyDocs/.videos/Discovery.avi' (explicitly force it to use a faster x11 driver with hardware pixel doubling), but you will get some black borders around image.

Currenly mplayer is good at playing video transcoded to 400x240, 400x224 (16:9) and 320x240 (4:3) resolution and is capable to play it at full frame rate.

Also youtube video seems to have 320x240 resolution, so it gets played fine by mplayer too. Used the following python script to download youtube files (though did not try it to run on the device itself yet ): http://www.arrakis.es/~rggi3/youtube-dl/

A better universal scaler that will work reasonably fast for any resolutions is planned in the future releases (along with YUV colorspace support).

As for the GUI, it would be really good to have it, any volunteers to create and contribute it?