How do I dial an extension number in Skype for Mac OS X?
To dial an extension number during a call, or to enter a number or character such as # on an automated phone system:
Click the Dial Pad icon. This is on the far right side of the menu bar.
Use the Dial Pad to enter numbers or characters.
Once the Dial Pad is visible, you can also use your computer’s keyboard to enter numbers instead of clicking each number with the mouse.