Sorry, that's right, it works. I forgot the definition. I have been trying
some things before.

In fact the problem is this:

\starttext

Testing

\startuseMPgraphic{gh}

textX:=87.51u;
textY:=96.31u;
pic:=thelabel.urt(btex {Test} etex scaled 1.00,(0,0));
draw pic shifted (textX, textY);

\stopuseMPgraphic

\useMPgraphic{gh}

\stoptext
And  I get:

! terminal: >> pic
>> picture
! Equation cannot be performed (numeric=picture).
<to be read again>
                   ;
<*> ...rt(rawtextext("{Test}") scaled 1.00,(0,0));
                                                   draw pic shifted (textX,
...
>> pic
>> (0.90863textY,textY)
! Not implemented: (unknown numeric)shifted(unknown pair).
<to be read again>
                   ;
<*> ...00,(0,0)); draw pic shifted (textX, textY);
                                                   ;
>> 0.90863textY
! Undefined x coordinate has been replaced by 0.
<to be read again>
                   withpen
draw->...:also(EXPR0)else:doublepath(EXPR0)withpen
                                                  .currentpen.fi._op_
<to be read again>
                   ;
<*> ...00,(0,0)); draw pic shifted (textX, textY);
                                                   ;
>> textY
! Undefined y coordinate has been replaced by 0.
<to be read again>
                   withpen
draw->...:also(EXPR0)else:doublepath(EXPR0)withpen
                                                  .currentpen.fi._op_
<to be read again>
                   ;
<*> ...00,(0,0)); draw pic shifted (textX, textY);
                                                   ;
.
system          > tex > error on line 14 in file kk2.tex: terminal: >> pic
>> picture
! Equation cannot be performed (numeric=picture).
<to be read again>
                   ;
<*> ...rt(rawtextext("{Test}") scaled 1.00,(0,0));
                                                   draw pic shifted (textX,
...
>> pic
>> (0.90863textY,textY)
! Not implemented: (unknown numeric)shifted(unknown pair).
<to be read again>
                   ;
<*> ...00,(0,0)); draw pic shifted (textX, textY);
                                                   ;
>> 0.90863textY
! Undefined x coordinate has been replaced by 0.
<to be read again>
                   withpen
draw->...:also(EXPR0)else:doublepath(EXPR0)withpen
                                                  .currentpen.fi._op_
<to be read again>
                   ;
<*> ...00,(0,0)); draw pic shifted (textX, textY);
                                                   ;
>> textY
! Undefined y coordinate has been replaced by 0.
<to be read again>
                   withpen
draw->...:also(EXPR0)else:doublepath(EXPR0)withpen
                                                  .currentpen.fi._op_
<to be read again>
                   ;
<*> ...00,(0,0)); draw pic shifted (textX, textY);
                                                   ;
 ...
 4
 5     \startuseMPgraphic{gh}
 6
 7     textX:=87.51u;
 8     textY:=96.31u;
 9     pic:=thelabel.urt(btex {Test} etex scaled 1.00,(0,0));
10     draw pic shifted (textX, textY);
11
12     \stopuseMPgraphic
13
14 >>  \useMPgraphic{gh}
15
16     \stoptext
17
<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.14 \useMPgraphic{gh}