It is possible to run Mist on a private net by issuing the following command. This binary is usually available in /opt/Ethereum on Linux (Ubuntu) platform:
Under Mac OS this is usually available under /Applications/Ethereum Wallet.app/Contents/MacOS
There are two methods of connectivity to geth from Mist, one is using IPC and the other is RPC/HTTP based. Both commands are shown here:
$ ./Ethereum Wallet --rpc ~/.ethereum/privatenet/geth.ipc
$./Ethereum Wallet --rpc http://127.0.0.1:8545
If you run Mist using -rpc with the HTTP option then it will display a message saying that it is a less secure method of connection, simply press OK as being on a private test network, as the network is local and not connected ...