I was hoping there would be more detail in the manual than there actually is. Look for packet filtering options in both the wan and lan settings. It's also possible given the large number of options I can just barely make out in the tiny pictures in the manual, that it has a setting to specifically block ping request (my own router does this)
Unfortunately since the manual doesn't walk through all the possible settings I don't think I'm going to be able to point you directly to it. If your router has firewall rules, you may simply try allowing for the wireless ip you are using to have full access.
I'm sorry I can't be more specific you're going to have to dig around a bit since the manual doesn't contain a full break down of the interface for me to track the setting down for you.