How to have my SSL certificate active on my check out page for marketpress

I am using both marketpress and membership. I have been able to get the SSL certificate to work for the membership plugin but not for the marketpress one.

How do I force or activate the ssl certificate on my checkout pages or any of the pages that the customer is inputting information?

Second Question:

I got a test account over at and used the test login id and trasaction number. I tried to use a test credit card number but kept getting " Your payment was declined. Please check all your details or use a different card. " when trying to create a subscription through the membership plugin. Is there any way to test it before making it go live?