(The solution is based on what was proposed here).
- Go do Programs and Features, select your Visual Studio installation, and run a Modify (if not present, you may be forced to go with Repair). Check the checkbox that mentions Silverlight
- Afterwards, when reopening Visual Studio to see if the project(s) is/are correctly loaded - you will be prompted to install Silverlight Developer - the pop-up will include the download link as well. Download, install, and you are (hopefully) done.