I'm using Ubuntu GNU/Linux 11.04 with Pd installed from a latest build .deb (it is confirmed on other machine running Ubuntu 11.04 too).
The iemmatrix is missing from /usr/lib/pd-extended/iemmatrix.
I've compiled and installed iemmatrix from the Pd SVN [1] and added /usr/lib/pd-extended/iemmatrix on Pd's File -> Path... and it worked.
[1] svn co https://pure-data.svn.sourceforge.net/svnroot/pure-data/trunk/externals/iem/iemmatrix
Anonymous
The correct path is /usr/lib/pd-extended/extra/iemmatrix instead of /usr/lib/pd-extended/iemmatrix
The package installed is from http://autobuild.puredata.info/auto-build/latest/Pd-0.42.5-extended-ubuntu-natty-i386.deb
A friend installed the same package and /usr/lib/pd-extended/extra/iemmatrix exists but it's not listed by default on Pd's File -> Path... So, just adding this path fixes this bug.
In the nightly builds of Pd-extended 0.43, only a couple libraries are loaded by default. You should load libraries you need in your patch by using either [import iemmatrix] or [declare -lib iemmatrix].