You can open the Dropbox upload dialog via one of the following methods:

Once reaching the Dropbox upload dialog, you can choose assorted settings that will be applied to the uploaded video. After making your selections click on Link to Dropbox..., this will open the Dropbox Authorization dialog.

Once the Authorization dialog appears, login to your Dropbox Account, and follow the prompts to allow the application to access your online profile.

After authorization is complete, you can click the Upload button to commence the conversion (if required) and upload process.