I am afraid that the issue is out of support range of VS General Question forum which mainly discusses
WPF & SL designer, Visual Studio Guidance Automation Toolkit, Developer Documentation and Help System
and Visual Studio Editor.
I am moving your question to the moderator forum ("Where is the forum for..?"). The owner of the forum will direct you to a right forum.
As far as I know, if you want to create Silverlight project, you need to install Silverlight Developer. Since you still get issues after installing it, I am afraid that there is anything wrong with the installation.