Xbox – get a veteran battlepack in bf4 when I played bf3 on xbox 360 and bf4 on pc

Yes, provided the videos are encoded in a format that the Xbox360 supports. Also, you'll need to install some software on your MacBook to serve the video. Connect360 and Rivet are commercial products do this. There's also a question over on Super User with some other options.

From the Battlefield Wikia:

Battlefield 3 is now the fourth game in the series to use the Battlefield Veteran system. If someone has played an older Battlefield game, they can receive the M1911 pistol in the game just by registering an old Battlefield game to the website www.battlefield3.com/veteran. It also unlocks two variants of the M1911, the tact and suppressed versions.

I'm not certain how one logs into the Veteran site to register other games, as the link provided just redirects to the BF3 site. Some folks apparently have 10 or more Veteran Points, and supposedly it's possible to make your veteran status show on your dog tags.