Hi igorlott,
the formats “avi” and “mkv” are containers. They can contain a lot af codecs.
Please write down the used codecs of the files, you can’t play.
To get this info, you can use mediainfo (http://mediainfo.sourceforge.net/).
Some media player (e.g. VLC) can show this info, too.
write something like
container: AVI/MKV
videocodec: ??? (e.g. xvid, divx, thedora, matroska)
audiocodec: ??? (e.g. mp3, ogg, wav)
WiiMC version: ??? (e.g. 1.0.2)
source: ??? (e.g. SD card, smb share or USB device)