The first thing you'll need to do is connect your Instapaper Account to Zapier.
You can then give your Account a title or label, which is just for your convenience and helps you track multiple accounts. It doesn't have any affect on which Account you actually add to Zapier.
You'll want to type in your Instapaper username and password. This gives Zapier permission to add things to Instapaper on your behalf.
If you typed your account details correctly you'll see a success message.
Now you can continue creating your Zap.