To connect your online store to your Merchant Account that is setup within Infusionsoft, you will need your Merchant ID. Here are the steps to locate your Merchant ID.
Go to E-Commerce > Settings
Click Orders from Left hand menu
Right click on the drop down located next to Default Merchant Acct for Manual Orders
If you are using Chrome the from your keyboard use Ctrl+F > in the Search field type "please select a default merchant"
User the number that is to the left of your Merchant
In Firefox you would follow the exact steps as listed above but the search is there automatically.