Download: http://dev-heaven.net/attachments/8193/_XerxesFixes.rar
BIS Thread: http://forums.bistudio.com/showthread.php?t=108206

Cut and past the @XerxesFixes into your main Arma2 directory.

Check armaholic here:
http://www.armaholic.com/plug.php?e=faq&q=18
If you don't know what a mod folder is.

For a mod launcher I suggest alpinestar's.

Mod PBOs:
Missile_fix_blu will make the TOW launchers (static, HUMVEE, Stryker, Bradley) on blufor tab lock.
Missile_fix_red will make the Reflex, Arkan, Metis Static and Konkurs (BMP2/BTR90) tab lock. SPG9 AT has longer range.
Missile_fix_BRDM will make the BRDM ATGM missile launcher tab lock.
RPKMagsForAK will make 75rnd RPK mags useable in AK74 and 107 of all types.
All are signed to my key.

Use this mod as you see fit, just give me credit if you modify it and release it ok? :P

Credits:
Thanks Imperator_pete for allowing me to build off from his arkan/reflex fix.
Thanks Wraith for helping me develop my scripting skills and answering my noobish questions!

Thanks for posting your Game Tweak addon release here
News is on the frontpage and you can find our mirror here:

Xerxes I appreciate your effort. However to me it seems you have misunderstood the config value.

Your change from 1 to 2 makes locking for the weapon available also in veteran and mercenary difficulty.
It has no relation to tab locking (friendly) units.

At least I was unable to get this to work and it is contrary to the docu of BI in the BIKI:

canLock: Defines if weapon can be locked on the target (white box appears)

#define LockNo        0
#define LockCadet    1
#define LockYes        2

http://community.bistudio.com/wiki/CfgWeapons_Config_Reference
http://community.bistudio.com/wiki/CfgAmmo_Config_Reference#canLock
http://community.bistudio.com/wiki/CfgVehicles_Config_Reference#canLock

So your addon is still welcome for those who want the change.
Just it seems that your change description is not fully correct.

I'm afraid I don't get what you mean? The mod makes it so that the weapons under the changes are capable of tab lock in all modes. I made the missile changes mostly with the aim of making it so that you have some idea if your ai is actually aiming at the target, rather than having to guess. Also, the BRDM ATGM weapon is bugged so AI can't use it atm, so this is a "workaround" of sorts.

Though if you could help me mate, i'm trying to make a furhter tweak so the T55 can tab lock things. However In all of the configs i've looked at, comparing it to the T34, I can't see the variable(s) that I need it change. By scrpting over the ZIS gun onto a T55 in a mission I know it's something to do with the weapon/ammo.cfg's not the vehicle.cfg . Please help?

Further info here: http://forums.bistudio.com/showthread.php?t=108199