You need to get lame > v3.90 installed first.
libmp3lame.so is searched in /usr/local/lib
( no time to write configure scripts ).
if it's installed elsewhere, change the Makefile,
you won't die from that.

untar in /my/pd/dir/externs

cd /my/pd/dir/externs/mp3cast~

make -f Makefile.linux

make -f Makefile.linux install

you're set !!