The Shipping Plugin Manager found in the Extend menu allows the administrator control over which shipping plugins will be allowed on the site. Customers will only see the enabled shipping options.
The Custom Shipping plugin only enables and disables the Custom Shipping, there is no configuration here. You have to go to Config > Custom Shipping to set your configurations.
FedEx, UPS and USPS Shipping
You will need information from your account at either FedEx, UPS or USPS in order to use these options. Simply enter the required information and publish the plugin to enable it as a shipping option when your customer purchases products.