Version 2.0 is currently only available to existing license holders, as it completes it final testing phase before public release. (Early Adopters program)

If you are not yet ready for Isadora 2.0, or just want to test the most recent version, you can download the pre-release here: http://troikatronix.com/download/isadora-pre-releases/
Although it will say 2.0 in the splash start up screen, it is only when you purchase a license or a license upgrade to 2.0 that the full feature set of 2.0 is unlocked. In the meantime you will be working with a beta version that has the latest bug fixes, but without the new features unlocked.

If you are a total newbie (welcome to Isadora!) you may want to consider working first with a demo version of the standard release, since it matches the pdf manual tutorials and existing tutorial videos.
Standard Release v1.3.x: http://troikatronix.com/download/isadora-download/
Be sure to download the manual from the 'Related Downloads' section, also run the installer for Pete's FreeFrame plugins, and visit the online video tutorials available via the help menu, all of which is available with the free demo version.

Isadora 2.0 has begun its roll out to early adopters. (Current license holders can upgrade early, before the official release).
Complete with publishable mapping points, that then appear as input parameters on the outside of the Projector actor, unlimited bezier curves and mirror options, and unlimited slices, alpha channels, and a lot more Just about everything you might need. New feature requests are invited from all users via the online support page.
Here is a teaser link to the online Projection Mapping beginners tutorial for 'IzzyMap' the new interactive projection mapping feature included in Isadora 2.0.
http://troikatronix.com/support/kb/projection-mapping-in-isadora/

Existing users of Isadora have been invited to register as Early Adopters of Version 2.0. Follow the link for details on how to update. New features include the built in IzzyMap projection mapper, Javascript actor, massive speed increases for HD playback, FFGL plugins, and plenty more. Currently only existing license holders can apply for early adoption of this version. The full public release will be coming along soon, at which point a demo version will become available for non-license holders to try out.