Adding column to Postgres table fails
The alter table was probably waiting to get an exclusive lock on the table. If you have transactions touching the table, the ALTER TABLE will wait until all those transactions are committed 

You can check if your statement is waiting for such a lock by looking at pg_stat_activity and pg_locks. 

If you have sessions that are shown as idle in transaction those are a potential cause for that.



Unrelated, but: Postgres 8.4 is no longer supported (=maintained). You should plan your upgrade to a supported version (9.3 or 9.4) as soon as possible. If you have to stick with 8.4 you should at least be running the latest 8.4 version which is 8.4.22