Morning
Ok, the first lesson you have unfortunately learned the hard way is to never remove a device form Sky Go unless you are definitely not going to use it again or you are told to by Sky. This is because you then hit the second issue of only being able to add one new device per calendar month.
So, you are a multiscreen customer which, as you rightly say, entitles you to free Sky Go Extra. However, you do need to activate it first. To do so, go to http://www.sky.com/shop/tv/sky-go-extra/ and click the link to add it. It can then take up to 24 hours to become active on your account.
A quick test to see if it is enabled is to see if Sky Go will allow 4 devices or two. Go to the Manage Devices section and it will show you. If only two slots show then it is not activated yet.