Installing plpgsql proc language

I've been trying to install the plpgsql proc language but the following
error keeps showing:

[whatever:/wherever] createlang plpgsql dbo
ERROR:  Load of file /Library/PostgreSQL/lib/postgresql/plpgsql.so
failed: no error message available

the plpgsql.so file exists on that location, i dont know what happen....

any help would be greatly appreciated (i run postgre under mac os x
10.2.6 client)

thanks!