How to add a mac address access control

> Model: C3700|N600 Cable Gateway Docsis 3.0

   C3700[v1] or C3700v2?  Look for "Model" on the product label.
Firmware version?  ISP?

> [...[ The Help Center at the bottom says there should be an "Enable
> Wireless Isolation" check box. But...there is no such field. Not sure
> what to make of this?

   "The Help Center at the bottom" of _what_?  Which page?

   I don't have a C3700[vX] to examine, so I know nothing, but, so far
as I know, the only relevant option would apply only to a "Guest
Network", namely, BASIC > Guest Network : Allow guests to see each other
and access my local network.  Otherwise, any device on your LAN should
be able to communicate without restrictions to any other.

> Are the [Mac] computers connected wired or wireless to the router? Are
> they on the same frequency or is one on 2.4Ghz and the other on 5Ghz?

   Unless a "Guest Network" is involved, none of that should matter.

> I'm trying to determine if my router has AP isolation which is
> preventing gameplay [...]

   I doubt it.  You might have more success if you described the actual
problem, instead of trying to implement some particular "solution",
which may have little or nothing to do with the actual problem.

   Can you "ping" these computers from each other, or do any other kind
of sharing between them (file, screen, ...), or any other kind of
communication, or is some particular (unspecified) game the only service
which is suffering?