There's actually a way you can link your gmail account to your Google Apps account - you still won't get the option to go to Plus from your domain e-mail hub, but you also won't get those annoying "This account is not authorized" type notes when you want to do something in plus, but have been checking your domain mail.

1. First, log into G+ or Gmail with your gmail login.
2. In the settings menu (gear icon, upper right), go to Google+ settings
3. Click on the menu item for Account Overview
4. For me, they changed this page (may be the same for others), so I had to click the link near the bottom that says, "Visit the previous version of the Google accounts screen "
5. On that page, near the top, look for "Multiple Sign-in"

Turning that on will allow you to fairly seamlessly go back and forth between your @gmail account and your @domain account. Once I did that and logged in under each, I haven't had too many problems getting right over to plus without having to log out and log back in under gmail.

Hope that helps...