Hi @Anonymous,
Welcome to the Fusion 360 Community!
This hung me up for a while too when I was first trying to upload meshes. The key note here is that in order to use meshes you need to be in a direct modeling environment as opposed to one that captures design history. There are two ways to do this, the first is to click "Create Form" in the create menu and this will bring you into the sculpt environment which is direct modeling or you can right click the top-level design in your Browser Tree (should just be the name of your project) and then select "Do Not Capture Design History". By doing either of these options you should then see Insert Mesh in your insert menu.
If this answered your question please feel free to mark this answer as a solution, that way others experiencing the same issue can benefit from this thread as well. If you have any other questions, don't hesitate to ask. I'm more than happy to help!
Thanks,
James Youmatz
Product Insights Specialist for Fusion 360, Simulation, Generative Design