When I tried to switch to testnet on Mist through develop->network->testnet, I met this problem, like the picture below. What's the problem? How can I fix this?
You need to start your
geth node first to connect to testnet, and then mist which will connect to the node. Instructions on starting
geth for testnet Ropsten Revival are here:
Blockchain explorer is here: https://ropsten.etherscan.io/
You can even use https://www.myetherwallet.com/ as a web wallet for the Rposten.