I have a problem with metapost when I write these lines:

label.llft(btex $0$ etex,(0,0));

the program don't know this message and it write
an error message:
This is MetaPost, Version 0.641 (MiKTeX 2.1) (preloaded mem=plain 2004.10.1) 2 OCT 2004 14:11
>> sinus.mp
>> sinus.mpx
! Unable to make mpx file.
l.26 label(btex $O$ etex,(0,0));
The two files given above are one of your source files
and an auxiliary file I need to read to find out what your
btex..etex blocks mean. If you don't know why I had trouble,
try running it manually through MPtoTeX, TeX, and DVItoMP.

Please can you help me ? Thank you...