When you click the icon in the app, and then click the Manage Ad Accounts button, is the ad account you want to add to Adview not displayed in the popup? If it isn't, there's one of two potential problems.
1. We have an old access token for you. To remedy this, click the Disconnect Facebook Account button under the settings menu.
Next, go to https://www.facebook.com/settings?tab=business_tools&ref=settings, and remove the Adview app:
After you've done that, go back to Adview in Hootsuite, and re-authenticate with Facebook. (you will be prompted to do so) Make sure you've selected all of the Ad accounts you want to use when re-authenticating with Facebook.
2. You don't have Admin access to the Ad account. If after doing #1, all of your accounts still aren't there, you do not have Administrator access to the ad account on Facebook. To fix this, you can either have the owner of the account give you access to it. (and then repeat the steps in #1) Or, have the owner of the account add Adview, and then have them share it with you by clicking the Share accounts under the settings menu. (the owner of the account should be a Hootsuite team administrator for this to work, which they usually are)