Once you have installed you get a 14 day trial of the premium features.
You can purchase at any time within the application.
After 14 days you will be placed on the free plan unless you have purchased.
The Campaign Monitor for Salesforce application is charged separately to Campaign Monitor (i.e. Campaign Monitor does the sending of emails etc, whilst Campaign Monitor for Salesforce manages the email data within Salesforce and the syncing of records between the two systems).
If you have any questions please click here