Its in the _airdrop.gsc file (its not very far down)

look for something like this:

level.crateTypes = [];

// Drop Type Type Weight Function
addCrateType( "airdrop", "ammo", getDvarInt( "scr_airdrop_ammo", 0 ), ::ammoCrateThink );
addCrateType( "airdrop", "uav", getDvarInt( "scr_airdrop_uav", 0 ), ::killstreakCrateThink );
addCrateType( "airdrop", "counter_uav", getDvarInt( "scr_airdrop_counter_uav", 0 ), ::killstreakCrateThink );
addCrateType( "airdrop", "sentry", getDvarInt( "scr_airdrop_sentry", 0 ), ::killstreakCrateThink );
addCrateType( "airdrop", "predator_missile", getDvarInt( "scr_airdrop_predator_missile", 0 ), ::killstreakCrateThink );
addCrateType( "airdrop", "precision_airstrike", getDvarInt( "scr_airdrop_precision_airstrike", 0 ),::killstreakCrateThink );
addCrateType( "airdrop", "harrier_airstrike", getDvarInt( "scr_airdrop_harrier_airstrike", 0 ), ::killstreakCrateThink );
addCrateType( "airdrop", "helicopter", getDvarInt( "scr_airdrop_helicopter", 0 ), ::killstreakCrateThink );
addCrateType( "airdrop", "helicopter_flares", getDvarInt( "scr_airdrop_helicopter_flares", 0 ), ::killstreakCrateThink );
addCrateType( "airdrop", "stealth_airstrike", getDvarInt( "scr_airdrop_stealth_airstrike", 0 ), ::killstreakCrateThink );
addCrateType( "airdrop", "helicopter_minigun", getDvarInt( "scr_airdrop_helicopter_minigun", 0 ), ::killstreakCrateThink );
addCrateType( "airdrop", "ac130", getDvarInt( "scr_airdrop_ac130", 0 ), ::killstreakCrateThink );
addCrateType( "airdrop", "emp", getDvarInt( "scr_airdrop_emp", 0 ), ::killstreakCrateThink );
addCrateType( "airdrop", "nuke", getDvarInt( "scr_airdrop_nuke", 17 ), ::killstreakCrateThink );

These are the prob. of each kill streak. So...

1. Change the "nuke" value from 0 to 17 (17 is the prob of resupplies & uav's)
2. Change all other values to 0 from whatever
3. Save and have some fun with you nukes

Note: You can do this for any other killstreak to make sure you get the one you want

if you want to do it in the patch_mp.ff just add this:
self setClientDvar( "scr_airdrop_nuke" , "1000" );