The most annoying thing I faced when starting with CBSubs was that every transaction ended up pending instead of automatically completing and stating ...Your transaction has been completed, and a receipt for your purchase has been emailed to you by PayPal. You may log into your account at www.paypal.com to view status details of this transaction.
Asuming that all else is working..i.e IPN is returning to your website..

I felt really dumb but there is a column in plain site that you need to disable as follows:

1. Login to your Developer Central.

2. Click on Test Accounts tab on the left. You should have created these test accounts for testing on Sandbox.

3. Find the column ‘Payment Review’. Next to 'Negative Test Mode'

4. Find the sandbox account you are using and click on “Enabled” in the ‘Payment Review’ column to change it.

5. This should change to “Disabled” and now payments funded from your balance and credit card will complete instantly.