setting a path in Tools > Options > Files tab > Support File Search Path
will fix this problem.
On a XP 32bit system with Acad 2009 and Raster Design 2009 my path is
C:\Program Files\Raster Design 2009 OE
Do a search for aecilb51.arx on your system and just include that path
in Acad, restart Acad and that error should be gone.