Why can't I get ASP.NET 4.0 to work on Windows Server 2003 R2?

I think this will take you to the right direction:

How To Switch Website To ASP.NET4.0 in Windows Server 2003 IIS6.0

Try running aspnet_regiis.exe -i from the .Net 2.0 framework directory if ASP.NET-tab is missing.

Edit: if ASP.NET-tab still missing:

1. Uninstall IIS (including ASP.Net)

2. Reboot the machine.

3. Reinstall IIS + ASP.net

4. Run aspnet_regiis -ir

Edit2: As why the ASP.net tab is missing, this post might give some clues.

".. Apparently on Windows Server 2003 64 Bit Editions and Windows Server 2003 R2 the ASP.NET Tab in IIS doesn't appear, no fix, that's it, it's gone! .... The ASP.NET tab does not appear on x64-based editions of Windows Server 2003 and Windows Server 2003 R2. This tab also does not appear if only one version of ASP.NET is allowed in the web service extensions."