Hi i1wan,
I tried the exactly same steps as you showed but I can't see "network & security" tab in vcenter web client even I logout and login again.
1. the time on vcenter server and nsx is the same:
vcenter server time:
localhost:~ # date
Tue Jun 14 13:55:35 UTC 2016
NSX time:
vc-nsx-180> show clock
Tue Jun 14 13:55:33 UTC 2016
2. all service is running on NSX:

3. NSX has regiestered to lookup service and Vcenter correctly:

but I can find "network & security" tab in vcenter web client even I logout and login again:

I'm so confused with that result because I did every step as the VMware doc said and has all service work fine.
BTW, i'm using VMware-NSX-Manager-6.1.2-2318232 and VMware-VCSA-all-6.0.0-3634788.
Is there's compatibility issue there?
Thanks in advance,
Please save me , god!
Mike