You can get the accounts defined in System Preferences like this:
ACAccountType *twitterAccountType = [_accountStore accountTypeWithAccountTypeIdentifier:ACAccountTypeIdentifierTwitter];
[_accountStore requestAccessToAccountsWithType:twitterAccountType options:nil completion:^(BOOL granted, NSError *error) {
if(granted == NO) return;
self.osxAccounts = [_accountStore accountsWithAccountType:twitterAccountType];
}];
Yan can then let the user select the ACAccount
instance she wants to use.
See STTwitter OS X demo project for a working example.