[metapost] MPtoPDF problem with LatexMP package?

[Maurizio]

I have the following MP file:

%%%%%%%%%%%%%%%%%%%%

input latexmp

setupLaTeXMP(textextlabel=enable,
class="article",
options="10pt",
inputencoding="latin1",
preamble="\newcommand{\pp}{\ensuremath{{{\mathbf p}}}}");

prologues:=0;

beginfig(1);

pickup pencircle scaled 1pt;
draw (0,0)--(2cm,0)--(2cm,2cm)--(0,2cm)--(0,0)--cycle;
label.top(textext("$\pp$"), (1cm,0));

endfig;

end

%%%%%%%%%%%%%%%%%%%%%%%%%%%

This file uses the LatexMP package and runs OK through MP (included in 
MikTeX under Win2K).

If I run MPtoPDF.exe on this file the program complains about not 
understanding the \documentclass control sequence which is found in the 
mpx314 file, created by latexmp (I guess). Clearly plain tex is running 
and not the latex format. However, if I hit "x" the program goes on and 
I get a correct pdf file. I have an older installation of all this on a 
different PC and I do not remember this problem happening before. Is 
there something which should be changed? I made some tests and the 
problem can be reproduced with a much simpler file (it's not related to 
the prologue statement etc.).

Thanks,
Maurizio