I have enabled to sync the frequent contacts on the Blackberry Manager. But how come I still can't find any contacts on my blackberry device? Is there something i should do after enabling? Pls advise. Thks!

Address book changes made in GW are scanned for every 4 hours. So if you make a change to which address book you are syncing with, wait 4 hours.

If you have waiting 4 hours then your address book might have corruption in it. Check the GWPA log, search for your name from the bottom up and see what it says. If the address book shows as unsupported then there is corruption. Run GWCheck.

is there a way of changing this from 4 hours
or manually pushing the address book

You can manually sync it by selecting 'Sync groupwise system address book' from the server configuration tab in Blackberry Manager. It is under Service Control & Customisation.

That syncronizes the system address book, not the subscribed address book.
If the address book on the BlackBerry is empty go into options and turn the wireless sync off, save and back on.