If you want to watch on Roku, you have to subscribe to the Roku channel.
As it is a separate Add On service you will not be able to log into the Roku Channel with your current account, you will need to create a new Passionflix account in Roku and pay an additional $5.99.
You also have the choice to cancel your current subscription to Passionflix and resubscribe via Roku for $5.99 a month. That subscription would be through Roku.
The channel can be found by searching for Passionflix on Roku.