You can start with test payments before your Merchant Application is approved.
You will receive email with verification link -> click it:
Log in your sandbox account and go to API settings:
Create and verify password, add your domain's IP address (you can use THIS service to define it), click 'Request API credentials':
When API credentials are created copy your Username and Password from API Credentials and paste them to your WordPress admin area -> Settings -> Payments -> BlueSnap:
Use THESE test card numbers to test your purchase.
Fill in your Merchant Application to accept live payments.
Step 1: Fill in your contact and business information.
Step 2: Fill in your banking information.
Step 3: Fill in business owner information.