Depending on the preferences, the widget will appear on the rightmost end of the 3D viewport header (you can drag the header with the middle mouse button in case it goes off screen), and it can also appear in the viewport sidebar (the N panel), in the tab called "Grid".

In case you have one or more add-ons that also add widgets to the viewport header and you want the Collection Grid widget to always be at the rightmost end of the header, please turn on the Add After Scene Load option.