I just downloaded the latest version of Electrum (2.6.4) and without thinking, sent coins to an address on the default_wallet. I've never used Electrum before but from my understanding, it's supposed to set you up with a wallet - password and seed - when you install it, right? That never happened for the default_wallet, and now I have coins on it that I can't send because it requires a password that I never had the chance to set up. What do I do?
That's strange, it should have provided you with a seed at the first run after the install and you must have entered the password whilst making the wallet, either that or there was a previous version of electrum on the machine and a wallet was made with a password on that.
Without the password and/or seed, there's not much you can do.