The undying creature will return to the battlefield before the Proliferate activated ability resolves, so you have the choice of it having one or two +1/+1 counters.
When you activate Plaguemaw Beast's activated ability, you pay all costs, includes the cost of sacrificing a creature. The ability is added to the top of the stack, and all costs need to be paid, just like casting a spell . Since the sacrificed creature died during the process of paying the costs for Plaguemaw Beast's activated ability, the undying creature's triggered ability will trigger. The undying ability will be added to the top of the stack, and will resolve before the Proliferate ability. If no one chooses to respond, the top of the stack resolves returning the undying creature to the battlefield, with a +1/+1 counter on it. Then, if no one chooses to respond again, the Proliferate ability resolves and you can choose to give the newly returned undying creature another +1/+1 counter.
From the Basic Rulebook:
. ACTIVATED ABILITIES - ... Each activated ability has a cost, then a colon (“:”), then an effect. (page 12)
. CASTING A SPELL - To cast a spell, take the card you want to cast from your hand, show it to your opponent, and put it on the stack. (The stack is the game zone where spells live. It’s usually in the middle of the table.) ... Now check what the spell’s cost is. Tap your lands to produce the mana necessary to pay that cost, and pay it. Once you do that, the spell has been cast. (page 10)
. TRIGGERED ABILITIES - A triggered ability is text that happens when a specific event occurs in the game. ... Each triggered ability starts with the word “when,” “whenever,” or “at.” You don’t activate a triggered ability. It automatically triggers whenever the first part of the ability happens. The ability goes on the stack just like a spell, and resolves just like a spell. (page 12)
. THE STACK - Spells and abilities exist on the stack. They wait there to resolve until both players choose not to cast any new spells or activate any new abilities. Then the last spell or ability that was put onto the stack resolves, and players get a chance to cast spells and activate abilities again. (You’ll learn more about casting spells and activating abilities in the next section.) This zone is shared by both players. (page 8. Also see an example on page 11)