If Mishra's Foundry is already a creature, activating the second ability will override any previous effects that set its power and/or toughness to specific values. Other effects that are affecting its power and/or toughness, including +1/+1 counters and the effects of spells like Giant Growth, will continue to apply.

If Mishra's Foundry becomes a creature and you haven't controlled it continuously since the beginning of your most recent turn, you won't be able to activate its first or last abilities, and it won't be able to attack (unless it somehow gains haste).