Scheduled Reminders - differentiating by contribution type
If I understand you correctly you have selected 'Activity' as the 'Entity' in the first drop down list.  Instead of 'Activity' you should select 'Contribution Type'.  You will then be able to choose the relevant financial types and contribution status (Ctrl-click to select more than one).

See the screenshot below (taken from the CiviCRM 4.7 demo site):


It sounds like you have already looked at the User Guide, but for the benefit of others here is the relevant page for more details: Scheduled Reminders

Please note that this functionality was added in CiviCRM 4.7.  It is not available in earlier versions of CiviCRM.  As you are using CiviCRM 4.6 perhaps you could consider upgrading to 4.7?