I am using http://blogs.microsoft.co.il/blogs/justguy/archive/2009/01/19/wpf-combobox-with-checkboxes-as-items-it-will-even-update-on-the-fly.aspx?CommentPosted=true#comment
Check the permission for your resource.resx file. At top of your file you will find the dropdown known as Access Modifier : change its type to 'Public'