Try setting the Ops class to be overloaded:
> setMethod(Ops, c("TestM","TestM"), function(e1,e2) {
+ e1@M + e2@M
+ })
[1] "Ops"
attr(,"package")
[1] "base"
>
> M = Matrix(diag(1:10),sparse=T)
> M+M # > FINE
10 x 10 sparse Matrix of class "dsCMatrix"
[1,] 2 . . . . . . . . .
[2,] . 4 . . . . . . . .
[3,] . . 6 . . . . . . .
[4,] . . . 8 . . . . . .
[5,] . . . . 10 . . . . .
[6,] . . . . . 12 . . . .
[7,] . . . . . . 14 . . .
[8,] . . . . . . . 16 . .
[9,] . . . . . . . . 18 .
[10,] . . . . . . . . . 20
> M+M # (NOT error)... was ERROR
10 x 10 sparse Matrix of class "dsCMatrix"
[1,] 2 . . . . . . . . .
[2,] . 4 . . . . . . . .
[3,] . . 6 . . . . . . .
[4,] . . . 8 . . . . . .
[5,] . . . . 10 . . . . .
[6,] . . . . . 12 . . . .
[7,] . . . . . . 14 . . .
[8,] . . . . . . . 16 . .
[9,] . . . . . . . . 18 .
[10,] . . . . . . . . . 20