i am trying to connect a client who is 15km away from the base station with a 411rb and a r52 wireless card but it does not see any of the interfaces at the base station. Do u think i can make it?

Yes, no problem, but you will probably need better antennas. what antennas are on both ends now?

5ghz at 8dbi

you need better antennas, look at the calculator: http://www.mikrotik.com/test_link.php,
it shows that your link is not possible until you increase antenna gain

something like this will make it possible: http://routerboard.com/pdf/ric522c.pdf (22dbi)

or you could switch to R5H cards and then even a 14dbi antenna would be enough.