As suggested by @Ramhound, I uninstalled everything related to VS 2012 and 2010, then installed 2010 + SP1 + applied Async CTP3. Properly installed Async CTP3 shows a welcome screen and prompts to restart. After that VS 2010 starts supporting the async
keyword. However, for the full support, it is required to reference a corresponding library from here (platform dependent):
c:\Users\%username%\Documents\Microsoft Visual Studio Async CTP\Samples\
In my case it was AsyncCtpLibrary.dll
. Otherwise it will highlight the syntax, but won't compile.