Thank you. I check it, and you are right. I forgot to include the
definitions when I cut and paste that part of the code. I thought the
problem was related to the btex command, but it's not.

The first line is:

u=1.000mm; linecap:=butt; linejoin:=mitered; path pp; picture pic;
labeloffset:=0;

I compared it with your code. The "numeric u;" definition is missing in my
file. I have included it and now it works!!

It's really weird since I compiled it in 2009 and it worked as is. I still
have the pdf output file...??

Finally I have a problem in this line:

externalfigure "aciddrops1.png" xscaled 80.13u yscaled 60.09u shifted
(69.85u,20.30u) ;

Then I got:

 terminal: >> picture
! Isolated expression.
<to be read again>
                   ;
<*> ...3u yscaled 60.09u shifted (69.85u,20.30u) ;
                                                   ;
.
system          > tex > error on line 99 in file kk2.tex: terminal: >>
picture
! Isolated expression.
<to be read again>
                   ;
<*> ...3u yscaled 60.09u shifted (69.85u,20.30u) ;
                                                   ;
 ...
 89     fill pp--cycle withcolor (0.000,0.000,0.000);
 90
pp:=(150.24u,73.96u)--(152.93u,72.91u)--(153.10u,74.30u)--(150.24u,73.96u);
 91     draw pp--cycle withpen pencircle scaled 0.30mm withcolor
(0.000,0.000,0.000);
 92
 93     setbounds currentpicture to
(67.85u,18.30u)--(197.56u,18.30u)--(197.56u,102.23u)--(67.85u,102.23u)--cycle;
 94
 95     externalfigure "aciddrops1.png" xscaled 80.13u yscaled 60.09u
shifted (69.85u,20.30u) ;
 96
 97     \stopuseMPgraphic
 98
 99 >>  \useMPgraphic{gh}
100
101     \stoptext
102
<inserted text> ...re.mp ; fi ; ;;;;]===], false)}

\processMPgraphic ...e ;\!!es , \MPaskedfigure )}}
                                                  \egroup \placeMPgraphic
\d...
\handleuseMPgraphic ...hics \processMPgraphic {#3}
                                                  \endgroup
\douseMPgraphic ...{\@@MPG \currentMPgraphicname }
                                                  \empty \endMPgraphicgroup
<to be read again>

l.99 \useMPgraphic{gh}