In your Homeworld 2 folder navigate to "bin/Profiles/Profile 1/Campaign/ASCENSION/ and find the "persist#.lua" files (the # is which mission the file is for and is replaced by a number from 1 - 15). For example, If you want to cheat on mission 3 you would use "persist3.lua" Open the file with TextEdit.app. At the very bottom of the file should be "RUs =" followed by a number which you can change to alter the amount of money/resources you want to have inside the game. (NOTE! you must collect some resources in game for it to allocate you your 999999 money. I wouldn't recomend using more than an 8 figure amount, it tends to cause bugs.
All Missions Cheat:
In your Homeworld 2 folder navigate to bin/Profiles/Profile 1/ and find the "playercfg.lua" file. Open the file in TextEdit and Find "maxmission = #" the number the # represents is the mission number you have played up to. Change the number to 15 to be able to play all missions.
Changing files may cause the game to not function properly. Make a backup before editing any files.
Thanks to Revolution Reader HereticXander!