If your subscription comes with Lync included, you do not need to add Lync to your hosting plan. However, if you are looking to add Lync services to your existing Exchange subscription, you can do so by following the steps below.
1) Log in to your Customer Control Panel, navigate to the Account tab and click on Buy Additional Resources.
2) Select the subscription you would like to add Lync to.
3) The list of subscription resources appears, showing all of the resources that can be added to your account. Select the number of Standard Users and Enterprise Users. Then click Next at the bottom of the page.
Note: You need to add one User per person that needs access to Lync. You can add more users later.
Standard users have access to IM, audio/video calls, and file transfers. Enterprise users have all of this functionality, plus the additional feature of conferencing.
4) Confirm the details of your order and click Place Order.
Now that you have added Lync user resources to your Exchange subscription, you will be able to begin setting up Lync. The next step is to enable Lync for your users. You can enable Lync for your users who already have mailboxes set up. Please see this article for instructions on setting up mailboxes.