For developers: http://tswaddons.wordpress.com/tio-developer-guide/
Please note that add-on developers need to change their code to hook into this - it's not something that "just works." So if you want this to happen with your favourite add-on talk to the developer and let them know you want it to support the Add-on Manager in Topbar Information Overload.
You open the Add-on Manager the way you used to open the Topbar Information Overload settings, it replaces that item in the menu. You can also Shift + click on Menu to directly open the Add-on Manager.
Inside the Add-on Manager you will see a list of registered add-ons. If you hover over an add-on in the list a tooltip will appear with some information. If it has options you can click on the add-on in the list to open its option window. It it's slotable you can Shift + click the add-on in the list to open the slot picker.
The slot picker will allow you to assign an add-on to any of the 8 slots in the topbar, so you can quickly access the functions setup by the add-on developer when clicking the icon. This is of course important for an add-on like Deck Manager, which would be a hassle to through the Add-on Manager to open it.
If the slot is dark it's already assigned to an add-on (a tooltip will reflect this and inform you of which one), but you can of course overwrite it - it's merely an indicator. Likewise the slots will light up in the topbar as you hover over them in the slot picker. If you want to remove an add-on from a slot simply open the slot picker for that add-on and click the slot it's assigned to.
In the Add-on Manager there is also a button for the slot settings as well as a checkbox which will close the Add-on Manager when you open the options for an add-on.
The slot settings window will allow you to configure the placement of the slots. Default they hook to the compass, which will give you the option of setting the icon spacing. You can also choose to move them as a group of 4 each, still allowing you to control the icon spacing. The last option allows you to freely move each slot to wherever in the topbar you want.
Note that if you input a number in the input box it will only get saved if you confirm the change with return/enter while the input box is in focus (your cursor is in it).