To make an MER in any of the toolkits, click on the "Firing" button and increase the "Quantity of Projectiles in Pod" to the number of weapons you want in your MER. Remember to increase the weight of the weapon, too. 6 bombs weigh a wee bit more than just one! Also make sure to take the weight of the rack itself into account (the standard US rack for carrying 6 Mk-82s weighs 500 pounds... and that's without a single bomb on it!)
Rapid Fire Packs
Let's say you just want to release one bomb at a time but at a steady, constant rate. For instance as you hold down the trigger, the bombs keep falling. Well it's simple.
1) Open your weapon in the TK and select Firing Info. Set Delay between game rounds to 0 and set 1 for Game rounds in a single burst and set your carpet burst to whatever you choose. That variable controls carpet bombing, as discussed much earlier.
2) Set the last line, Reload time between bursts to 0 and hit 'Okay.'
3) Open Ordinance Parameters and simply select Continuous Fire, then save and your bombs will be rapid firing.