I am able to use my google account to login to the web and the iphone app. But have downloaded the mac app and it the button doesn't work.
How best to solve this?
the button being the 'log in with you google account'
We're sorry for the inconvenience, we will address this issue in future updates of the Mac app.
For now, as a workaround, please try to log into your account through the web-app, go to Settings (top-right corner) -> Account and using the "Edit" link, set a password for your Todoist account.
Now, you should be able to log in using the regular Login button and typing your GMail address in combination with the newly set password. Note - you will still be able to log in using your Google account through the web and on the iPhone.
We've released a new version of our Mac app with bug fixes and new features such as quick task adding. Please install the latest version from http://todoist.com/mac and let us know if it fixes any issues you've been experiencing with the old version.