Hi,
The iPhone and Mac have to have been set up for iMessages to sync.
The iPhone must be joining the LAN where the Mac is using it's WiFi connection (the Mac can be Ethernet).
Then the Text Forwarding needs to be enabled on the iPhone in Settings > Messages > Text Forwarding.
It is best to do this when the Mac version is open as a Code is generated in a Pop up.
The Code number needs to be entered back on the Phone.
IF this is done already then the Text Forwarding section should list the devices it sis set up for and whether they are still active.

9:19 pm Sunday; November 22, 2015
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad