The csv files in the itemCollections directory are in a four column layout.
The first line of the file is the name of the collection.
subsequent lines are in the four column format.
Column 1 contains the name of the asset which may be the id of an item or the id of another collection.
column 2 is the type of asset.
AmmunitionBox for ammo
HeatSink for heatsinks
JumpJet for jumpjets
Upgrade for actuators, cockpit mods, gyros, TTS
Weapon for weapons
Mech for complete mechs
MechPart for partial mechs
Reference for other itemCollections
Column 3 is how many are available
set to 0 for unlimited or an integer for other quantities
Column 4 is the chance an item will be available with 0 being never and 10 being always

Add a new line to the csv file containing the asset name, type, quantity, and chance using the format above.
ex:
mechdef_atlas_AS7-D,MechPart,0,10

This info is also contained in the mod description flie

Other authors who would like help with this may contact me