AO Modern, AO Classic (When it released) and AO Grey all are now carbon copy styles except the colours and graphics may vary to suit.
The wau to send a pager is by either going to the button on every thread as pointed out above, going to someones profile by clicking on their name and selecting "Send Message" from under their user name.
The other way to send an Instant Message (They are not called pagers
) is to use your quick links menu and select "Send Instant Message" this will then bring up a list of people that are online and you select using a tick box who you wish to send the message to. You can also just allow messages to be sent to and from people on your friends list by enabling this within your profile.
Hope this helps.
PS: For those that cannot send a message, it is likely you have not yet signed up to the AO club.