You need to send an email to Trial Pay. When you click on "Complete offers" it'll take you to Trial Pay's list of offers you can complete. It says, "Popular, Free, Mobile, Surveys," etc. In the same line, but on the right, it says support. Click on Support and find the offer that you haven't been credited for and file a ticket through Trial Pay.
If you can't figure it out, under the spoiler is a screencap that can help.
Be prepared to provide proof that you did fully complete the offer, such as the last four digits of the card you used and/or the email that confirms you fully completed the offer. This goes for your other thread too about Free Credit Report. Just have the proof and send in a ticket to Trial Pay.
Then you should be credited if you fully completed the offer.