Redmine

Hello,

i want that redmine creates tickets when a new mail is coming in the exchange inbox of the user "support".
redmine can sends mails already. but i don't understand this howto:
http://www.redmine.org/projects/redmine/wiki/RedmineReceivingEmails#Fetching-emails-from-an-IMAP-server

I want to use this IMAP method. When I run the following in my redmine-1.3.0-1/apps/redmine folder where the Rakefile is located...

rake -f Rakefile redmine:email:receive_imap RAILS_ENV="production" host=192.168.0.31 *the ip of the exchange* username=support password=xxx

i get the following error:
rake aborted!
Could not find rails (= 2.3.14) amongst [rake-0.9.2.2]

what can i do?
thanks in advance!!