Windows 8 store app Dropbox get access token and upload issue

I see two immediate issues:

1. Your Authorization header on the call to /access_token seems to be missing the required parameter oauth_token. You seem to have it in the URL instead, which I don't think works. (I don't think you can mix parameters in the query string and in the Authorization header.)
2. You're escaping the signature, which causes it to be invalid Base64. (It ends with %3D instead of =.)

Also, I removed your app secret from your post, but others may have seen the post while it was visible, so you should consider it to be leaked.