Adding USAePay Extension

Hello,

I have an older version of OpenCart (version 1.5.1.3), and I have purchased and downloaded the USAePay Payment extension. I followed the instructions to upload it to my server, and all of the files seemed to transfer without a problem. However, when I log in to my cart and go to Extensions --> Payments, the USAePay module is not there. There is no change to the list.

Is there a step that I'm missing? Any help would be appreciated.

Walter Kaechele