Connecting sequence to table column

I am using PGSQL 8.2.3 on Windows XP and pgAdmin 1.6.3. While adding columns to a table from pgAdmin front-end, an
optionappears "sequence" with a drop-down box on the column creation window, but it remains disabled. I have already
createda sequence but want to connect that sequence with this column. How can I do this? Why this disabled drop-down
boxnot enabling?