See this Google help article on how to enable Google+ for your domain.
As a summary, you need to ensure that Google Talk & Picasa Web Albums are enabled before you can switch on Google+. Also, if Other Google Services is not available you may have to migrate your organization to the new account infrastructure.
I would also suggest reading the article in full as there are some considerations to take in to account when enabling Google+
To enable Google+
Log in to your Google Apps administrator control panel.
The URL is https://www.google.com/a/cpanel/primary-domain-name, where primary-domain-name is the domain name you used to sign up for
Google Apps.
Click Organization & users.

In the pane on the left, choose the organizational unit for which you want to enable Google+.
You can enable Google+ for only specific users, using organizational
units. Learn more
Click Services.

Ensure that the following services are turned on for the organizational unit:
Google Talk, which is listed under Core Google Apps suite. If Google Talk doesn’t appear there, you’ll need to add
the service to your organization’s Google Apps account. Learn
more
Picasa Web Albums, which is listed under Other Google services. If you don’t see the Other Google Services section,
you’ll need to migrate your organization’s Google Apps account to the
new accounts infrastructure. Learn more
Under Other Google Services, turn on Google+.

Click Save changes.
It may take some time before Google+ is available to all users and
they are able to sign up for the service.