Time machine problem

[roshdi80]

Hi, this i smy configuration :
Linkstation duo raid 1 firmware 1.64
Imac os 10.7.5 (wireless connection to nas)
Mac book air os 10.8.1 (wireless connection to nas)
I can read and write in all the folder from the nas without any problem but I'm not able to use a folder for a time machine backup.
I create a folder called TM, activate from Network Setting --> AFP, and from extension I enable time machine with the folder TM.
I see the TM folder on the desktop, but from mac System preferences --> time machine I can't select the folder TM.
Any idea to fix the problem (if it's possibile to use time machine with lion).
Thanks in advance for your help.
Angelo