I had exactly the same problem...i had the dll in the privateassembly folder of c:\program files\microsoft visual studio 8\common7\ide\privateassemblies..it kept on giving the same error message as you have.
I trawled the web and found a reference to installing it in privateassemblies of
c:\program files\microsoft visual studio 9.0\ .....privateassemblies instead
and then it worked...bobs your uncle
my question is what is this visual studio 9.0 folder ? is this also installed when vis studio 2008 is installed ??? Why does this ssrs projects developed in vis studio 2008 recognise this privateassemblies folder of this one and not the one of microsoft visual studio 8...Any thoughts guys ?