I guess I must have been stupid, because the issue look so simple.
I have checked:
1. the mirror server has packages of my arch (armel) and updated.
2. deb-multimedia-keyring is installed.
3. the very package exists on mirror.
What I get:
$ sudo apt-get install ffmpeg-dmo
Building dependency tree
Reading state information... Done
E: Unable to locate package ffmpeg-dmo
I did update immediately before install attempt:
$ sudo apt-get update
Hit
http://www.mirror.realss.com stable InRelease
Ign
http://cdn.debian.net squeeze InRelease
Hit
http://ftp.jp.debian.org unstable InRelease
Hit
http://cdn.debian.net squeeze Release.gpg
Hit
http://cdn.debian.net squeeze Release
Hit
http://www.mirror.realss.com stable/main Sources/DiffIndex
Hit
http://www.mirror.realss.com stable/main armel Packages
Hit
http://ftp.jp.debian.org unstable/main Sources/DiffIndex
Hit
http://cdn.debian.net squeeze/main armel Packages
Hit
http://www.mirror.realss.com stable/main Sources
Hit
http://ftp.jp.debian.org unstable/contrib Sources/DiffIndex
Hit
http://ftp.jp.debian.org unstable/non-free Sources/DiffIndex
Hit
http://ftp.jp.debian.org unstable/main armel Packages/DiffIndex
Get:1
http://www.mirror.realss.com stable/main Translation-en_US
Hit
http://ftp.jp.debian.org unstable/contrib armel Packages/DiffIndex
Hit
http://ftp.jp.debian.org unstable/non-free armel
Packages/DiffIndex
Hit
http://ftp.jp.debian.org unstable/contrib Translation-en/DiffIndex
Hit
http://ftp.jp.debian.org unstable/main Translation-en/DiffIndex
Hit
http://ftp.jp.debian.org unstable/non-free
Translation-en/DiffIndex
Fetched 4,460 B in 39s (113 B/s)
Reading package lists... Done
And that the .deb packages do exist on the mirror, verifiable here:
http://www.mirror.realss.com/deb-multimedia/pool/main/f/ffmpeg-dmo/
Below is my /etc/apt/source.list. What could be wrong?
$ cat /etc/apt/sources.list
deb
http://cdn.debian.net/debian squeeze main
deb
http://ftp.jp.debian.org/debian/ unstable main contrib non-free
deb-src
http://ftp.jp.debian.org/debian/ unstable main contrib non-free
deb
http://www.mirror.realss.com/deb-multimedia/ stable main
deb-src
http://www.mirror.realss.com/deb-multimedia/ stable main
