to_char function for 'timestamp' and 'datetime' type?!!

Hi, all:

PostgreSQL for Linux 6.5.3.

Our app need to use 'to_char' function on one 'datetime' column, I use
as following:

'select to_char(col_name,'MM/DD/YYYY HH:MI:SS') from tab_name where
...... ;'

The error is 'No such function 'to_char' with the specified
attributes.'

Now I try the 'timestamp' type, 'cause our app has to show both date and
time in one single columns.

'create table try (try timestamp);' createing is OK.
'insert into try values ('04/13/2000 00:00:00');' inserting is also Ok.
'select to_char(try, 'MM/DD/YYYY HH:MI:SS') from try;' Same error as
above.
'select to_char(timestamp 'try', 'MM/DD/YYYY HH:MI:SS') from try;' the
error is 'Bad abstime external representation 'try'.

I have no idea what to do now! According to the docs, 'to_char' function
should work on 'timestamp' data type. But I cannot make it work, Syntax
error?

The other thing, 'cause we cannot create index on 'timestamp' columns.
We may have to stick with 'datetime' type. But how can we use 'to_char'
function on 'datetime' data type?

Could anyone help us out? Kind of emergency

Thanks!!!

Jack