According to Blake Shepard, Senior Business Planner Lync, Lync-Skype connectivity is included in the normal Lync Standard CAL.
“What Lync licenses are required for Skype Connectivity? Skype Connectivity is licensed as part of Lync Standard CAL, Lync Online Plan 1, Lync Online Plan 2, or Lync Online Plan 3.”
Technically you will need to setup Live Messenger PIC. Here is How: