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