We just installed the plugin for version 3.70 and as simple as it may be to install it, getting it to work required some help, which we received from the plugin developer Harout, and another nopCommerce user mahoney_brat. Hope that these tips will help you if you are getting "object not set to an instance of an object" error, or getting the error page when clicking on the Estimate Shipping button.
For the plugin to work, you have to make sure that Shipping Origin fields are all filled in. Go to Configuration >> Settings >> Shipping Settings.
Use the register button of the Shipping plugin and complete the Canada Post account set up steps. This will fill in the values of the textboxes which are set to readonly so you might not be able to just type those values in. Event if you already have an account use the register button and log in to existing Canada Post account, then follow the steps.
It appears that ContractID field is not required and might be left empty by the set up process, if you don't have a contract with Canada Post.
Again thanks to mahoney_brat and Harout for your help.