I run Wireshark on a MacBook Air and I also have a Android phone on the same WiFi-network. If I visit a website with the Android phone I can't see that DNS-traffic (or other traffic) in Wireshark on my MacBook Air. Is there any way I can capture all traffic on the WiFi-network in Wireshark?
You'd need to capture in promiscuous or monitor mode; not all Wi-Fi adapters and their drivers handle promiscuous mode, but many of them handle monitor mode, including the adapters in various flavors of MacBook and the OS X drivers for them.
If the network is encrypted (WEP/WPA), monitor mode traffic will be encrypted, and you will have to arrange that Wireshark can decrypt it.