This how to assumes you have already read this post: http://www.alanpinnt.com/2012/06/29/how-to-get-wepay-api-credentials/
If not I recommend you do.
1. Extract the plugin
2. Copy both folders directly into your OpenCart Installation, just drop it in there.
3. Once both folders are uploaded, login to your OpenCart admin login.
4. From there go to “Extensions > Payment”, Scroll down to WePay and click install on the right.
5. Now that it is installed you will click “Edit” on the right again.
6. You should now have a page that needs your API credentials, fill in the details and hit submit.
7. Test and confirm its working.