ArcGIS 10 Compatibility
update2
Arc2Earth V3 is now compatible with ArcGIS 10.0!
Download the latest version here: http://www.arc2earth.com/products/download/
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Arc2Earth V3 is currently not compatible with ArcGIS 10.0. The 10.0 extension framework is very different the the one used in V9.0-9.31 and will require a new compiled version of Arc2Earth. Up until now, we have tried to maintain a single installer/binary file that can be used with all versions of ArcGIS, at 10.0 it looks like we'll need to break from that format.
When the 10.0 version of Arc2Earth is ready, we'll make it freely available to all current users
update While Arc2Earth is still not completely compatible with 10.0, there is a script you can run to make the A2E Toolbar appear. As of now, this is unsupported but much of the functionality appears to work (with the exception of the Map Tile Layer toolbar)
Here is the ESRI document for installing older extension with 10.0
http://resources.arcgis.com/content/kbase?fa=articleShow&d=37639
The command line for Arc2Earth would be:
%CommonProgramFiles%\ArcGIS\Bin\ESRIRegAsm /p:Desktop "%CommonProgramFiles%\Arc2Earth LLC\Arc2Earth\KMLExporter.dll"
This will eventually be added to the Arc2Earth installer so the registration will happen automatically