1. Firstly you need to login into your Dropbox account. Then go to Developers page
2. Click on Create your App
3. Set Dropbox API, Full Dropbox, name of app and click Create App button
4. Then your new app profile opens.
Put this link to OAuth 2 Redirect URLs
5. Then bring these parameters
and put it in vTiger CRM MultiLoad extension settings page
Parameters saves automatically by ajax, you need just unfocused field.
Then you need to click on Login button to connect vTiger MultiLoad extension to Dropbox.
(!) Remember that you need to have ssl certificate on you vtiger crm server, it means that you crm should opens by https:// link.
And on config.inc.php file you should have $site_URL with https:// too.