You can either link up the modules yourself like this:

ln -s /etc/apache2/mods-available/userdir.conf /etc/apache2/mods-enabled/userdir.conf
ln -s /etc/apache2/mods-available/userdir.load /etc/apache2/mods-enabled/userdir.load

Or you can use the Apache utility

a2enmod userdir

Just remember to restart the server with

/etc/init.d/apache2 force-reload

Also, keep in mind that by default, the home directory for your user will need to require a public_html directory.

/home/username/public_html