go into your IIS manager on the Workflow server, and expand your server at the top-left
click on Applications Pools
on the right, you should see all your applications pools, and the Identity they're using
you may see some like ApplicationPoolIdentity; these should be configured as actual service accounts instead of the built-in defaults like this. for example, "ProcessManagerPool" is likely your default that's being used by Workflow, or perhaps the "DefaultAppPool". right-click on the proper application pool and select Advanced Settings. "Identity" can be found in the third section. click in the right field and click the ellipsis (...), and choose Custom account, and "Set..." this will allow you to set a specific service account to which you've given the proper rights to access the directories you're attempting to manage via Workflow.