To read the unnamed (default) registry value, enter the key path without name (with trailing backslash character). For example: "Software\Microsoft\Windows\"Bearing this in mind, I followed these steps to specify a (Default) value for a key in the MSI I am building via Visual Studio:
1. Right Click on the Setup project -> Select View -> Registry
2. Construct the path to your registry key. In this case, I was creating a new key for the AWSSDK assembly as HKLM\Software\Microsoft\.NETFramework\AssemblyFolders\AWSSDK.
3. Right click on the AWSSDK node and select "String Value"
4. Leave the "Name" field blank in the properties window but specify the correct value; in this case, it was the installation path for the SDK.
When you have entered a value, you will notice that the empty name field gets replaced by (Default).
This is a good explanation of why there is a (Default) value for registry keys: Why do registry keys have a default value?
Also, the Microsoft Orca tool is a great aid when debugging MSI issues. You can get Orca by downloading and installing the package from :here: