SBCL 1.0.12
CL-USER> (require :parenscript)
NIL
CL-USER> (ps:ps (in p o))
"p IN o;"
CL-USER>

"in" should be lower case.

I tried both the tar version parenscript-20071104 and the latest version at http://common-lisp.net/project/parenscript/repository/parenscript/

andy peterson