How to Fill a series

I want to add a column and fill each cell with a unique number starting at 1 and proceeding to end of spread sheet data. I believe this is a "Fill-Series" function. I use to do this at my old job but when I go to "Fill-Series" the series is grayed out. I have treid highlighting the entire column etc with the same result. I must not be addressing this in the correct method. Thank you for your help.

Fred Ullrich

I've never seen this problem, but a couple of things to try before you are driven to insanity:

Try typing a "2" under the cell where you have the "1"
Highlight both and fill that down. See if it "un-greys" your Fill Series.

Last ditch effort: If you can not get your Fill Series to work, for what ever reason, and A1 holds your "1", then A2 could be a formula of (A1+1), which you can drag down to the end of the list.