Can anybody provide details on how to access the Marketo Sandbox?

Hi Adam,
You'll have to contact your sandbox admin and ask them to invite you, the same way you would be granted access to the normal work environment.

The previous administrator has left the company........

Do you know the URL to access the sandbox?

In the past, Marketo did not give sandbox environments to all of their clients.  You might have to reach out to them directly and make sure you have a sandbox environment.  They could then tell you who the admin is of that environment and they could get you access.  

We reached out to our rep when we were having trouble locating the user credetials for our sandbox. He worked with support to get us an invite to the account so that we could remove the previous admin and access the instance.

The URL to acces the sandbox is the same as what you login to for your main instance, https://login.marketo.com/homepage/login. You will need support to add you as a user if you don't have one and the previous admin did not leave you credentials.