The current version of EMP, which I believe you're referencing, already includes the "login-as-customer" feature (that's what the Place Order button does). I'm not sure what additional functionality you were looking for in installing this one.

That plugin used to automatically login to the selected customer's account until DrByte pointed out that the feature essentially gave any admin-hacker the keys-to-the-kingdom, so the functionality was removed. If you choose to restore it, you can see how it was done in an older version of EMP.