How do I work with Client.AccountInfo?

Are you by chance using https://github.com/dropbox/dropbox-js? (In general, it would probably help if you told people what library you're actually using.)

Assuming you are, AccountInfo is a class. You'll need an instance of that class.

I think you're looking for code like this (completely untested):

client.getAccountInfo(function (err, accountInfo) {
    if (err) {
        console.log("ERROR: " + err);
    }
    else {
        console.log("Email: " + accountInfo.email);
    }
});