Youtube – Why can’t I enable YouTube as a service on the Google Apps Free account

This used to be available, but has been discontinued:

Note: The previous Start Page service was a syndicated version of iGoogle; it allows you to set up a dynamic homepage (one start page layout per domain) for your users that brings together your content, Google services, and the best of the web. Starting on 3 December 2008, the Start Page is no longer included for new organizations that sign up for Google Apps.

(From: http://www.google.com/support/a/bin/answer.py?hl=en&answer=46798)

However, you can now make your own via Google Sites:

As a Google Apps administrator, you can create a Start Page for your users, using the Google Sites Start Page template. This page is your users' homepage, on which they can access their Google services and other content -- such as interactive gadgets and RSS feeds -- when they log in to their Google Apps accounts.their Google Apps accounts.

(From the tutorial: http://www.google.com/support/a/bin/answer.py?answer=179916

I see YouTube listed in the dashboard, under "Organization & users" > "Services":
https://www.google.com/a/cpanel/my.domain/Organization#Organization/subtab=services

This is described in the help center.