you can get the official iemmatrix.dll from:

ftp://ftp.iem.at/pd/Externals/IEMMATRIX

there is another dll, that somebody compiled, because the official did not work on his platform (amd sempron K8):

iemmatrix.dll

binary for osx:

iemmatrix.pd_darwin

sources from cvs:

http://pure-data.cvs.sourceforge.net/pure-data/externals/iem/iemmatrix/