In order to always find the latest update/install files for Silverlight Developer Runtime (especially the 64-bit version), you can go to the following link:
Microsoft Silverlight Release History
On this page, click the link for the most current release (typically the first link under Silverlight 5).
Then click the link to the KB article for this release (eg. "KB4023307" or whatever the current KB is).
On the KB article, you should see methods to obtain the updates. Click the link for the Microsoft Update Catalog (usually Method #2). This will provide a list the appropriate entries from the update catalog. Click the "Download" button for the Security Updates option.
You should now get a window with a list of download options. Click the link to download the "silverlight_developer_x64_blah...blah.exe" version or any other download you may need.
This is a number of steps but the only way I have found to reliably download the current developer runtime for x64.