AVI to MP4

[randogrulz]

I run Mageia and have a # of videos that are AVI and to view them on Roku I need to convert them to MP4. I have the vid out as COPY audio out AAC (lav) out format MP4 MUXER. i go to file then open and choose my video click open once it finish with its exercises I then click on save  put it in the folder I created  and all is good. I did maybe 35-50 videos and everything worked fine. Then I did another video and it did its exercises but when I go to select file to save up on the name line it doesn't show the file. I don't know what I did wrong. A friend told me to go to command line but I want to do it in a GUI not  a shell. any suggestions?
thanks
randog

[eumagga0x2a]

Then I did another video and it did its exercises but when I go to select file to save up on the name line it doesn't show the file.

I can't make heads or tails of this. Avidemux doesn't prefill the location bar in the file dialog? Are there some special characters in the name of the source AVI video? What keeps you from providing a filename even if none is auto-suggested?