I have a 3750x 48port POE switch that I installed a second power supply in. But it doesnt power on. Am I supposed to enable it in the switch somewhere? I've plugged it into a couple different outlets so I know that's not the problem.
Are you sure the power supply you've just installed is OK?
Every 3750X I've plugged a second power supply into it just worked.
Is your new supply rated the same as your existing supply (I.E. are they both 350 watt, or both 712 watt, or both 1100 watt supplies)? If it's smaller than the in-situ supply (I.E. existing supply 1100 watts, new supply 715 watts), and the existing supply is using more power than the new supply is rated for, this may prevent it from turning on.
You don't need to do anything. If the power supply is good when you connect the power cable to it, it will turn on just like the first power supply. It also doesn't matter if the first power supply is 350w and the second one is 715w or the first one 715w and the second one is 350w. I have 3750s with 2 different types of power supplies with no issues.