Assign Custom product fields to select products

Hi, just installed Business edition for a client, never had a need for custom product fields, now I do.

I managed to create the custom field ok, but not sure how I get it to be "Assigned" to selected products only (Not across the board, can it be done by category?) and how to make it display on the product page and then into checkout etc?

Thanks Nicolas, was making it of the type product so it was not showing up.

Don't suppose there is an option to have the custom field appear depending on the selection of an option?

Like...

Personalised message - yes (Show Field)
Personalised message - no (Don't show field)

Nope, one is an option as it needs to add a small cost.