The Test Merchant Option is already a default merchant on your Merchant's page when you signed up for NexusMerchants.
It will allow you to create a dummy merchant account and perform a test transaction to see how it will work before you start buying your first merchant. This looks as shown below.
clicking on the pencil icon you get the keys which you have to put in your Infusionsoft by Keap app. and you need to choose Nexus merchants as you merchant.
Once you are happy you can subscribe purchasing by clicking on the big PLUS of the Add Merchant Icon under the merchant tab from the menu bar
Follow the indicated steps from the wizard.
This Link may be helpful
When you wanted to view the transaction history on this specific merchant, you can simply click on the eye icon and it will route you to the right page.