If a card says, "When attacking...", does that mean at any time during the attack? Or does it mean as soon as the ship is attacking?

For example, Zeta Leader says, "When attacking, if you are not stressed, you may receive a stress token to roll 1 additional attack die."

Do you have to decide to use this ability right away (once you've declared a target?), or can you wait until the last possible moment?

The "when attacking" is a generic timing window, meaning than the ability should be resolved somewhen in the attack sequence. The exact time depends on the ability you are using. Check out the attack timing chart in the FAQ (page 8), it will tell you exactly what abilities can be resolved in each step.

In your example, Zeta Leaders ability triggers in step 2ii:

Quote

2. "Roll Attack Dice" step

i. Identify number of attack didce (from Ship cand or secondary weapon)

ii. Resolve abilities that increase or decrease the number of attack dice (including Range Combat Bonus)

As Ubul says, it's a fairly generic timing window, but the text on the card usually narrows that window down. For example, Backdraft says " When attacking a ship inside your auxiliary firing arc, you may add 1 (CRITICAL) result. " The crit result would normally be added when the attacker can modify dice during the Modify Attack Dice step, specifically 3.ii. Zeta Leader, Mauler Mithel and Backstabber all trigger at step 2.ii of the attack. Wampa triggers at the start of the Compare Results step.