You won't, because not a lot of people have received their order yet.
You could try to use the configuration guide for the 3750X or you can raise a TAC Case.
But the feature is available and working on the C3850, right ?
I can't say yes or no because I don't have one.
All I can say is that 3850 can support MACSEC. As to right-now or after-the-X-release, is up to anyone's guess.
The best authority is to raise a TAC Case. The TAC engineer handling your case will "reach out" or "engage" the product engineers for more definitive answer.
The answer is No. They have been dangling this carrot in front of us for months. The documentation on the 3850 is confusing as it says the NM for the 3850 is natively capable of anything the SM on the 3750X including MACsec. This is true as the chipset on the module does support the feature. However they have yet to release a version of IOS XE for the 3850 that enables it. Check these release notes dated as of June 17th under the Important Notes section
It is available now: