You must not add the parenthesis after not.

This is not correct. You may even have to use parentheses unless the negated condition is a conjunction or disjunction. The omission of the parentheses is tolerated when the argument is a single Pattern.
-W

Try this:

rule "Flag missing Foos with values"
when
not Foo(stringProp=="blah", intProp==5)
then
System.err.println("A proper foo does not exist");
end

Bye
Raf