You need to have Skype Credit or a subscription to activate call forwarding.
In Skype for Windows, click Tools > Options > Calls > Call Forwarding and choose Call forwarding options to activate the feature.
In Skype for Mac, click Skype > Preferences and under the Calls tab, you can activate the feature.
Skype website or the Skype Call Forwarding website
Skype Call Forwarding User Guide