You will not be able to find a valid MAC address by looking at a tcpdump filter of any kind. The output merely confirms that the AP does not accept your injected packets, which means that there most likely is a MAC filter in place.
No it is not. You will have to use a valid MAC address in order for the AP to respond to your injected packets. The only way to get hold of a accepted one in case you do not know it by default would be to intercept one as soon as a client connects, or by bruteforcing it using MDK3. Since you do not seem to know the MAC address the question why inevitably springs to mind, knowing the whole scenario would make it easier to point you in the right direction.



