piechartMP installation:

Copy the files shown on the left side in the directories
shown on the right side and update the file database LS-R
if necessary. 

piechartmp.mp  --> TEXMF/metapost/piechartmp/
LEGAL, README  --> TEXMF/doc/metapost/piechartmp/
piechartmp.pdf --> TEXMF/doc/metapost/piechartmp/
/examples/*    --> TEXMF/doc/metapost/piechartmp/examples/

TEXMF means the root of your system TEX tree, e.g.:
  C:\texmf\               (win)
  C:\texmf-local\         (win)
  /usr/share/texmf/       (Unix)
  /usr/local/share/texmf/ (Unix)
  /usr/share/texmf-local/ (Unix)
  /opt/texmf/             (Unix)
You should prefer "local" paths for the installation.
Please consult the documentation of your TeX-system.