You can find it by right-clicking Crash Fixes in your mod list, selecting "Open in Explorer," and navigating to the SKSE/Plugins folder. Essential Settings to Enable
Changing a setting here to enable OSAllocators forces the game to stop using its internal crashfixplugin.ini
For UseOSAllocators to work, the d3dx9_42.dll from the SKSE Plugin Preloader must be in the main Skyrim folder. You can find it by right-clicking Crash Fixes
You will most commonly find crashfixplugin.ini in the root directories or Plugins folders of games like: selecting "Open in Explorer