Why can't I see my Mesh?

I am very new to Unity and I am trying to create my own Mesh.

I have created a simple quad with Modo and I imported it in my assets. When I choose to change the Mesh of one of my GameObjects, I can select my quad, but when it is applied to the GameObject, I can’t see the quad in the scene (unlike the Plane Mesh that I can see perfectly).

Any idea why?

Try flipping the mesh over. Meshes are one sided, and you may have the normals the opposite way with the mesh you created.