It's humorous how often this has come up... (Don't worry, I did it myself).

No one who has not already signed up can start a Mac account, or play on the official Mac Client with their previous account because it is only in Beta Testing Stage, and all the players who have been allowed to play beta have already been selected.
i.e. Beta Testing is closed to new players.

For those of you who try to login anyway, this is the reason it says your username/password are incorrect....

I assume you can all still play iLoL, but if you don't want to, then you'll just have to be patient. Still no word on a rough time line or potential release date. Beta testing has been going on for 2 weeks, but I still wouldn't expect the official release for at least another month, if not longer...

Whenever I try to sign in, it says that its the wrong password and username when I know that its not.... why is this? anyone help?

Caman guys look at what SableElephant just posted its not gonna be out for new users for a while calm down and be patient

I have a serious issue with the client!
Every time I open the client it takes a WHILE (a very long time) for it to show up. The actual client that is.
When it's loading all I see is the "League of Legends" logo and it's called "User-Kernel".

After 5-10 minutes it opens up, patching, or actual log in client.

I dont know why every time i try to login on Mac it always appear Login error "Invalid Username/password combination. please try again" my password and username is correct... if on windows, i can login.

So basically anyone who had an account for the Windows PBE can only play on the Mac PBE? i dont see why they dont open the Mac PBE since not every person who owns a windows owns a Mac.

It's supposed to be a public beta. I'm not sure whoever is running the show understands the idea of how public beta's work.
Preventing people with valid accounts from logging in because the haven't participated in another LoL beta or are long time players of the game is... well. For the lack of a better word. Stupid.
You're limiting your testing pool (which is not what you're supposed to do in a PUBLIC BETA).
You are not in a public beta. And calling that is an affront to beta's. It's a closed beta. This should be updated so people actually may want to participate don't waist their time unless they've been invited.

Either that or you need to remove the login restrictions and actually let us test this client.
One way or the other, you should make the "change" right away.

Surprisingly, this works extremely well on my 2012 Mac Mini. I have never tried to game on a Mac before so I had no experience coming in. Great job Riot! Now there's more than just Xcode on my mac! w00t!!!!

Oddly enough, every time I press enter to type, it automatically types "undefined".