As seems that the "Insert OID" button do nothing in DUDE 2.0b1, I see no way to put a RSSI level on link labels until a new release.
Also, its possible to make a probe using RSSI levels. As service, this probe can send a notification about a predefined level of the RSSI. The problem I see here is that you need to create a unique probe for every device you want to see RSSI from SNMP, cuz part of the OID number is the MAC address of that interface.
Then you cant have a general service to send notifications about RSSI levels. I didnt know why MT does not made that OID number together with the interface´s index instead of the interface´s MAC address. Another great thing MT should put in their SNMP is the number of total wireless clients connected to an AP. So far I found only RSSI levels that you can view at the "wireless registration table" item on the "snmp" tab.