One of my favorites MB Oktoberfest, brewed with US-05. Keep the temperatures around 65-68 deg F if you can, and give it the full three weeks. I tend to go lower on the carb sugar, but that is a personal preference.
only octoberfest i have done, was a true lager. used white labs german lager yeast and did the lagering steps fermenting low and raising to diacetyl rest....etc. you could easily use the ale yeasts you have on hand and it would taste just fine.
One of the most confusing aspects for new brewers. MrBeer products are brewed using an ale yeast. This is because the ale yeast does not require the strict temperature control that a lager yeast requires. The HME with the ale yeast will taste similar to an identical brew with a lager yeast. The intention is for you to brew beer with a taste within the style range of the labeled HME. If you're a stickler on semantics call it Oktoberfest Ale if you like. I have brewed it both ways and none could tell which was which.