Can I test this plugin before registering a Scotiabank account?

Yes you can:

1. Change the currency of your website to USD
2. Configure the plugin using the Test Storename: 811839488380015 and Test Shared Secret: f^31YA5p.AJ
3. Use either of Test Credit Cards
   1. 5204 7400 0000 2745
   2. 4004 4300 0000 0007
   3. 5426 0640 0042 4979
   4. 4147 4630 1111 0059

As well as any date in the future and any CVC code on the payment page

That's it! You have successfully completed a test transaction.