OK obvious first question to ask: does the addin extension onLoad run when you open ArcMap and then your MXD?
Did your add-in previously save information in this map document?In other words was OnSave called and you added data and then saved the mxd?I have not used this before for add-ins but have some experience with classic IExtension objects.
I had problems with OnLoad not executing when double clicking the mxd. Then I went to the config file and set the autoLoad attribute to true on the Extension element (default is false). I also found this was the case in ESRI's sample solution, but it is not mentioned anywhere when you view the sample code in the browser. There are a couple of bugs in the sample code as well (null references - really?).
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