Installing Keynote Tweet 2

Run the terminal app.
At the prompt, type: sudo gem i twurl --source http://rubygems.org
Enter your admin password (probably your usual OSX password)
Register a developer 0Auth application at http://dev.twitter.com/apps/new to obtain a consumer key and secret. The application name needs to be unique - I used KeynoteTweetforifenn. You can use what you enter here for the description field too. For website, I entered my personal homepage. As you will be the only person tweeting to your account, it shouldn't matter. Agree the terms, complete the captcha and 'Create your Twitter application'.
(And this is important.) Click 'Settings' on the tabs under your application name. Then change 'Application type' to 'Read and write'. If you do not do this, you will not be able to tweet. Click 'Update this Twitter application's settings'.
Go back to your terminal window and type: twurl authorize --consumer-key the_key --consumer-secret the_secret
Both the_key and the_secret are given on the twitter application page.
This will give you a web address in the terminal window. Don't press return or anything in the terminal window - this caught me out - but just copy and paste the web address into your browser and hit return.
The web address will return a pin code. Copy and paste this into the terminal window and hit return. Congratulations. Your mac can now tweet. :-)
Download Keynote Tweet 2 from http://splatdot.com/the-oauthpocalypse-and-keynote-tweet/
Unzip the zip file and copy Keynote Tweet 2 to your applications directory. Now run it and enter any hashtag you wish to use. Then run keynote - anything between [twitter] and [/twitter] within your slide notes will be tweeted.