It may be a function of the version of MediaLab you are using. Because changes were made to how ML handles custom items during the early releases of v2006, it's recommended that you use the final release of v2006 (available from http://www.empirisoft.com/dowload_old.aspx) or v2008 or later. And also to use the syntax recommended in the final set of samples provided in v2008 (I've attached them here in case it helps).
See also:
http://www.empirisoft.com/support/sh...ad.php?p=12953
http://www.empirisoft.com/support/showthread.php?t=1325