I did the following :
"You can drop the .so into /usr/lib/pidgin/ or ~/.purple/plugins/.
Debian (and Debian-derived distros e.g. Ubuntu) will also need to 'sudo ln
-s /usr/lib/libpcre.so.3 /usr/lib/libpcre.so.0' for this binary to work. "
as mentioned in FAQ no. 11 and restarted the pidgin app. But the
musictracker plugin still doesn't appear in the plugin list.
For more information:
http://code.google.com/p/pidgin-musictracker/wiki/FAQ