The Tweetbot for Mac alpha is set to expire today. Tapbots has released a beta to replace it, but it comes with a huge, Twitter-policy forced, caveat: You can't add any previously un-added Twitter accounts to it. Developer Paul Haddad explains why on the Tapbots blog:
You can grab the Tweetbot for Mac Beta here. It’s important to realize that with this build you can’t add or reauthorize any accounts. If you delete an account or de-authorize an existing account you will not be able to add it again until the final version comes out.
If you try to add a new account or launch the app with no accounts created you are going to end up right here on this page.
So you can replace the Tweetbot for Mac alpha with the Tweetbot for Mac beta, but you can't add any accounts to it you haven't added already, or replace any you remove in the future (until the final version is released in the Mac App Store). The reason for all this hooha is, of course, and the reason Tapbots had to remove the alpha download previously, Twitter's new, developer-hostile API 1.1 rules which greatly restrict the amount of user tokens any 3rd party app can receive. Ever.
That also means if you tried Tweetbot for Mac alpha, and didn't like it, you should got to Twitter.com and revoke access to it, and any other Twitter app you're no longer using, so the developers can re-use those tokens for future users.
You can grab the new Tweetbot for Mac beta via the link below.
Source: Tapbots blog