Hi,
The Process is:-
Start the app on the iPhone (it should now send iMessages from the iPhone Number).
Start the app and add an Apple ID in Preferences > Accounts on the Mac. (this can now send iMessages from the Apple ID)
Add the Apple ID to the Messages settings (Send and Receive) to link the two.
The iMessages should now appear on each device (ones to the iPhone Number on the Mac and to the Apple ID on the iPhone).
This will only sync from this point onwards.
If you have changed the iPhone or the Mac and have had to reset things then you may have "lost" iMessages in the intervening time.
For SMS (Text Forwarding) you have to have the iPhone on your LAN (join the home network by Wifi) and then Set the Text Forwarding setting on the iPhone (And wait) the Code number should appear on the Mac version to enter back on the iPhone (which is why you are waiting on the iPhone).
If your Mac and iPhone can do Handoff and Continuity then you may need to Enable Blue Tooth as well.
9:10 p.m. Thursday; September 17, 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