Hi Roi, are you signed in to AGO, under file-->sign in?
Hi Roi,
I've seen this issue when the Add-in Manager settings are changed from 'Load all add-ins without restrictions (Least Secure)'. Try the following:
1) Open ArcMap
2) Go to Customize > Add in Manager > Options tab > Select "Load all Add-Ins without restrictions"
3) Go to Customize > Customize Mode > Commands tab > Type in "ArcGIS Online" > Click and drag in the command "Add Data From ArcGIS Online" up to the Add Data button in the user interface