Capturing an Element can be done at any time, but takes a little while - one turn, when in combat. No roll is normally required on a willing or indifferent Element. If you are attempting to sneakily Of course, on some powerful, large, or particularly unique elements, the DM may ask you to roll a Challenge roll.
You cannot capture an element that is in combat against you, however the round after a creature is defeated, you may attempt to capture the creature with a CHA check as a standard action, you may also do this to creatures on your side [even temporarily]. However, elements captured this way will be considered unwilling.
If the DM references a philosophical or literary concept or work in the campaign, you may attempt to capture it. A CHA check may be required, but once captured the DM must write an effect and limitations for the element and give it to you.
Finally, some characters may have a concept or item that the player wants to use as a plot element. In these cases the Player and the DM can collaborate in creating the item, which the player will eventually earn in exchange for paying a certain amount of experience [either directly to teh DM out of character or to an NPC in game] or completing a sidequest.
Once captured, the Element may be summoned in combat, or to help with Challenges. Willingly Captured Elements will allow you to control them and dictate their actions when Summoned. Unwillingly Captured Elements, however, are more unpredictable. Each time they are summoned, the summoner must roll a CHA check, if they succeed the Element follows orders and will never require a CHA check again, if they fail however, the Element will use it’s effects randomly, or even in a way specifically detrimental to the Summon and his allies.
Also, remember that you have only so many Element Slots, determined by your Charisma. While Capturing an Element is a standard action, vacating a slot is instantaneous.