I recently encountered an issue when developing a BizTalk Server 2010 solution using Visual Studio 2010. When selecting to add a new item to a project, the schema item was missing in the Visual Studio dialog box:

I fixed this issue by repairing the Visual Studio 2010 installation as follows:
- Open Control Panel and select Programs and Features
- Find the listing for the edition of Visual Studio you are having a problem with and select it.
- Select to Uninstall/Change from the top of the program listing.
- In the Visual Studio Setup Wizard, select Repair/Reinstall and then select next.

- You may be prompted to restart you computer to complete the installation.

- Follow any instructions until the Wizard completes and then select finish. In my case I had an error related to SQL Server Compact Edition, but this is not installed when developing for BizTalk and is an optional component:

- Once the Repair process was completed the option to add a BizTalk schema in Visual Studio was once again available:
