This alert might appear when attempting to run Reason 3.0.4 or earlier on an Intel Mac. For Intel Mac compatibility, you'll need Reason 3.0.5. This is a free update for all registered owners of Reason 3, and can be downloaded from propellerheads.se
If you are already using Reason 3.0.5 and are still getting this error when you attempt to set up a ReWire session, its likely that your ReWire host application isn't Universal Binary. If this is the case, you will need to update your host software. The following versions of common DAW/host software are Universal Binary:

• Logic Pro and Logic Express 7.2.1 and later
• Ableton Live 5.2 and later
• Pro Tools HD 7.2.1, LE 7.1.1, and MPowered 7.1.1 and later
• Digital Performer 5.1 and later
• Cubase and Cubase Studio 4 and later