How do I add a mesh in a script?

Did you mean to add a mesh THROUGH a script, or add a mesh INSIDE a script?

The first one is done using this code :

M = Instance.new("BlockMesh")   -- The type of the mesh can be BlockMesh, CylinderMesh, SpecialMesh or a CharacterMesh
M.Name = "I'm a mesh"   -- You can alter a mesh's name by doing this
M.Parent = Workspace   -- This line will add the mesh directly below Workspace

Mainly that's how it works. The second one is impossible (I think)

mranderson11