Issues with adding a copyed rocket motor in ksp

[starky]

I love playing around with KSP i downloaded the latest version and having issues i tryed copying the lv-45 engine was wanting to test out modding the fuel flow i have tryed everything put it in the game data but not the squad folder.  i put mechjeb in and it works but when i put the new lv-50 it wont show up in sandbox when im building not in the engine list and cant get the read only off the ksp folder dont know if that matters.  Any ideas on why it wont show up?  i have put the folder by itself tryed putting it as parts engine also