Can you activate Weaponmaster at any time during your turn, or is it only when the unit is played?

You can only activate Weaponmaster when the unit with the keyword is played.

According to Rule 821.1.b, Rule 821.2, and the associated FAQs, Weaponmaster is a triggered ability that acts specifically as a "play effect." It is not an activated ability, and therefore cannot be activated at-will during your turn.

Key Details:

• Trigger Condition: It triggers specifically "When you play me" (the unit with the keyword).
• On-Board Functionality: Once the unit is on the board, the Weaponmaster keyword becomes inactive and has no function (Rule 821.2).
• No Free Equips: Because it is only a play trigger, it does not allow you to equip gear for free (or at a discount) at any other time. Any equipping done after the unit is already on the board must follow standard rules for the Equip keyword, which requires you to pay the full cost and can only be performed during your own turn in an Open State (with nothing else on the chain).

In summary, Weaponmaster is a one-time opportunity to attach equipment at a discount that occurs exclusively as part of the process of playing the unit.