First, you require AMXX to be installed. You can do this by installing AMXX via the mod central on the control panel.

Weapon restrictions are disabled on AMXX by default.

So, edit the following file:

cstrike/addons/amxmodx/configs/plugins.ini

or if you are using DOD

dod/addons/amxmodx/configs/plugins.ini

Find the line:

;restmenu.amxx           ; restrict weapons menu

Can you see the semi-colon ( ; ) ?

Remove it!

Reboot the server.

You should now be able to access the restrict weapons option on the AMXX menu in game.