how do you set foriegn keys in pgaccess?

I am a bit of a newbie to postgres, but I managed to install 8.0.4 on my
windows box and it mostly appears to be working fine;
I can set a primary key constraint, but when i try to set the foreign key it
requires a 'reference' - but there is nothing there to chose from.

I also have another query:
How do you export the database tables and relationships etc? I found a
pg_dump command in help and I am guessing this I what i need but where do I
enter this command in pgaccess?

Also how do you input existing SQL script into postgres?

Thanks for any help.
Jeremy

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