don't use database manager page for this. use pg tools with correct user credentials and you won't have any problem.

psql -h localhost -U username -d database -W < /path/to/dump.sql
password: ****

if you have any permission problems after importing the database from command line: you can fix them using the following commands on tables, views, sequences. good luck.

-- alter permissions [optional]

REASSIGN OWNED BY old_user TO newuser;

ALTER DATABASE old_user OWNER TO newuser;

Tables [ownership]

for tbl in `psql -qAt -c "select tablename from pg_tables where schemaname = 'public';" dbname` ; do psql -c "alter table $tbl owner to username" dbname ; done

Sequences [ownership]

for tbl in `psql -qAt -c "select sequence_name from information_schema.sequences where sequence_schema = 'public';" dbname` ; do psql -c "alter table $tbl owner to username" dbname ; done

View [ownership]

for tbl in `psql -qAt -c "select table_name from information_schema.views where table_schema = 'public';" dbname` ; do psql -c "alter table $tbl owner to username" dbname ; done