Install from the DVD instead of the deployment - there appears to be an issue with the registration of .dll files when they are installed from a deployment.
Manually register the APIContext.dll file by doing the following:
- Close Revit 2010
- Open the Windows Command Prompt
- Switch to the Revit MEP 2010 directory
- Type RegSVR32 APIContext.dll
- Close the Windows Command Prompt
- Launch Revit 2010
Autodesk is aware of the issue and they are looking into the cause. Stay Tuned! When we know more we'll post an update.