Well, are you using 1Password with a plugin, or just the site? I generally use it with Chrome.
So, first install the plugin.
Then goto the autofill passwords area and make sure chrome uses one password for passwords.
Now, when you click the 1Password button next to the URL it will open a new tab where you can login to your account.
After you enter your master password, close that window.
You should now be able to go to a username or password field and press down arrow, and it will prompt you with appropriate passwords for that site, if you have more then one login item for a URL, make sure you label them with names that tell you which account is which.