in the rules.ini:

[Mods]
NewAircraft=
NewBuildings=
NewBullets=
NewInfantry=
NewUnits=LaserTank
NewWarheads=
NewWeapons=MINI_LASER

;1 Mod Unit
[LaserTank]
BaseType=LTNK
BuildLevel=5
ScenarioLevel=5
Prerequisite=NONE
PrimaryWeapon=MINI_LASER

;2 Mod Weapon
[MINI_LASER]
BaseType=OBELISK_LASER
FriendlyName=
Projectile=LASER_BEAM
Damage=50
RateOfFire=40
Range=1024

gives the error: Failed to parse value 'MINI_LASER as weapon for [LASERTANK -> PrimaryWeapon]: String not recognised as a Weapon type: MINI_LASER

1) I also tryed to copy the mod weapon below the weapons category in the rules.ini

2) I also tryed to write it in the xml:
<?xml version="1.0"?>

<!--
The commands in this file enable all infantry/buildings/units to be buildable
when the rules.ini is configured properly
-->

<ModGameCommands>
<CNCModGameCommandManager>
<CNCModGameCommands>
<!-- mods -->

<Command Name="LaserTank">
<GameCommand>GAME_COMMAND_TD_MOD_CUSTOM_BUILD1</GameCommand>
</Command>

<Command Name="MINI_LASER">
<GameCommand>GAME_COMMAND_TD_MOD_CUSTOM_BUILD2</GameCommand>
</Command>


</CNCModGameCommands>
</CNCModGameCommandManager>
</ModGameCommands>