In

http://yt-project.org/doc/examining/loading_data.html

import yt

units_override = {"length_unit":(1.0,"Mpc"),

                  "time_unit"(1.0,"Myr"),

                  "mass_unit":(1.0e14,"Msun")}

ds = yt.load("id0/cluster_merger.0250.vtk", units_override=units_override)

It seems a : is missing after time_unit

Yingchao