Nothing happens in KeyChain Access when I try to open a .cer file

I am trying to install a iOS development certificate to my keychain. When I double click the downloaded .cer file, or drag it over the Keychain Access icon, Keychain Access opens, but nothing else happens. I don't even get any error messages. I have tried to restart Keychain Access, and restart the computer. I also tried it with another certificate to see if there was something wrong with this one, but I got the same result. What can I try?