If you import an extension but you already have an app open, you need to press Ctrl + F5 to refresh the screen and reload the Qlik Sense interface. With that action, it will refresh the custom objects list with the new extension.
To import an extension in Qlik Sense Enterprise Server, you need help from the Qlik Sense Administrator. Only a Qlik Sense Admin can import an extension.
You can find out more information about this at https://help.qlik.com/en-US/sense/February2018/Subsystems/ManagementConsole/Content/import-extensions.htm.