In 1945 my mother-in-law and her family were forced to leave
Grodislanitze, Germany, now Poland. the government of the day
expropriated their land and home and told them to flee. They all
immigrated to Canada. My husband was born in that town and would like
to file for compensation for the land they left.
In what office can I file for compensation for their land that was lost?

It appears that no compensation will given. Here is a link to an
article on Polish reparations in connection with WWII.

http://news.bbc.co.uk/2/hi/europe/3692444.stm

Long story short "Germany and Poland have agreed to set up a joint
team of lawyers to thwart any claims for reparations in connection
with World War II. " In other words both Germany and Poland will be
fighting this one.

You should note that the article is dated September 27th, 2004

Didn't get to post everything I wanted there....

"Emotive issue 

The claims by a small group of Germans for ancestral property, lost
when Poland's borders shifted westwards after the war, have caused
huge resentment in Poland.

One of the groups - the Prussian Claims Society - has said it will
take cases to both Polish and European courts.

That prompted the Polish parliament to pass a toughly-worded,
non-binding resolution calling such claims groundless and urging the
government to take up the matter with Berlin.

For most Poles, the issue of moral responsibility is clear. 

For its size, Poland suffered far more damage and casualties at the
hands of the Germans than any other country. Six million Poles - half
of them Jews - were killed in the conflict. "

Hope that helps

immigrant1943-ga,
I live in Germany and agree with elids-ga. The article seems accurate.
Since Poland joined the European Union last summer, the two
governments have been moving to settle all WW II and subsequent open
legal problems.
As you can imagine, there are a lot of people here in your father's
position, and there was an outcry last fall, but it has died down
since.
Sorry.