Dropbox Sync API access token

I'm not familiar with the term "access key," but if you're asking about an "access token," that's a term associated with OAuth 2, while the URL you have is dealing with OAuth 1. In OAuth 1, you get an token (oauth_token) and a token secret (oauth_token_secret). If you need to convert that to an OAuth 2 access token, you can use the /token_from_oauth1 endpoint.

That said, I'm pretty sure the version of DropNet you're using only supports OAuth 1, so you should probably stick with the token and token secret that you have. (Your code looks good to me up until the call to GetAccessToken.)