Enabling your Costco/Elavon Virtual Merchant account to allow for web based transactions:

Login to your account @ https://www.myvirtualmerchant.com/VirtualMerchant/login.do

In the top menu, mouse over "Terminal" and then mouse over "Advanced". 

Find the "Enable HTTP Transactions" checkbox and check it, then click Update. 

The next step is to set up your Costco/Elavon Virtual Merchant account in your ISN:

Log into your ISN account

Go to Settings -- Office Settings -- Payment Options

Select VirtualMerchant(Elavon) and enter your current Account ID, User ID and Current Pin. The Account ID is the ID you use to login to your VirtualMerchant account. The User ID is the User account in your VirtualMerchant that has been given the appropriate permissions to conduct web based credit card payments. The Pin # is the password for this User account. 

Once that is done scroll to the bottom and click Save Payment Preferences
Now, your ISN should be able to process transactions through your merchant account!