unit step and dirac functions



Hello,
I'm trying to plot a simple signal with Maxima, I have found resources 
claiming that unit_step(x) and delta(x) functions exits for Maxima, in fact 
they do not (at least for me, or I couldn't manage to use them). Here is my 
input and the result:

(C1) plot2d( unit_step(x), [x, -10, 10] );

*** - COMMON-LISP:FLOAT: ((|$unit_step| SIMP) -10.0) is not a REAL
The following restarts are available:
USE-VALUE      :R1      You may input a value to be used instead.
MACSYMA-QUIT   :R2      Macsyma top-level

1. Break [1]> 

I would be very very happy if you can help. If you can CC to me, that would be 
better.

Thank you very much.
Dogac SENOL