i've spent the last week or so trying to start developing an add-in for the desktop version of microsoft OneNote, but all i've been able to find in terms of documentation are add-ins that seem to run exclusively as task-pane add-ins for the web client. is there something i'm missing? where should i look?
Not exactly so. Most probably you have created a new web add-in based on the task pane. But web add-ins may include dialogs, ribbon UI customizations and etc. Read more about possible things in the OneNote add-ins documentation.