One other thing to note here, if you are not getting the grey hovering button to add new tags you may be using the old invoice template. The tagging system is only supported on the newer invoice templates (introduced in August 2013). You can check by going into the sales menu then selecting invoice customisation from the dashboard.
If you are on the old system you will see a large banner at the top with a link to upgrade.
After upgrading it should then work fine!