Documentation helper gives an error
Running AlgebraExample
Error in mxRun(modelD) :
MxAlgebraObjective's objective algebra does not evaluate to a 1x1 matrix.

I'd suggest this:

A <- mxMatrix("Full", nrow = 3, ncol = 3, values=2, name = "A")

Simple example: algebra B simply evaluates to the matrix A

B <- mxAlgebra(A, name = "B")

Compute A + B

C <- mxAlgebra(A + B, name = "C")

Compute sin(C)

D <- mxAlgebra(sin(C), name = "D")

Make a model and evaluate the mxAlgebra object 'D'

A <- mxMatrix("Full", nrow = 3, ncol = 3, values=2, name = "A")
model <- mxModel("AlgebraExample", A, B, C, D )
fit <- mxRun(model)
mxEval(D, fit)