portscan2 problem

["reynald" <rtm () cybees com>]

hello,

I am using snort 2.1.0 ruleset. I know that preprocessor "flow-portscan" replaces "protscan2". 
My question is, how can I run portscan2 again? 

I tried adding "preprocessor portscan2: <options>" in my snort.conf, but it prompted an error that it doesn't 
recognized portscan2.

Did I missed something?

note: I have also activated preprocessor conversation (according to the manual its a prerequisite).

thanks,
reynald