There's no doubt they'd get a full stadium, wherever they play it - plenty of people in the baseball community across most of Europe. And all the American businesses here would take the corporate seats and boxes in no time at all. The question is, which stadium to play in?
I think one of the reasons Amsterdam is favoured is the Amsterdam Arena, which has a retractable roof so there's no risk of losing the game to a rain-out. However, it's a bit difficult to see how it could be set up for baseball - the playing area is relatively compact for soccer, and doesn't have a lot of extra space around it. If they do squeeze baseball in, then it's going to be an odd-shaped field, likely with a large screen in left or right.
If London is preferred, then the game will be outdoors and at risk from the British summer weather. Quite where in London they would play would be up for debate. Wembley Stadium would seem to be too big at 90,000 capacity and with the same layout issues as Amsterdam. Arsenal's Emirates Stadium is even more compact in playing space, although a better capacity. I think more likely would be the Olympic Stadium, currently being converted for soccer use by West Ham. However, it will retain the ability to be converted back to hosting athletics, and thus has a much larger playing area than a conventional soccer stadium. This would look a better candidate for conversion to baseball, although West Ham don't move in until the start of the 2016/17 season
The other option would be to use one of the cricket grounds. Lord's is the largest but also the "Home of Cricket" and I can imagine the old blazers who run the MCC would be very skeptical about allowing baseball to be played there. Although it would be quite fun to see the old pavilion being used as the clubhouse. Home plate would probably go bottom left of the photo below, between the Mound stand and the UFO-esque media centre
And then the other cricket option is the Oval, which lacks the capacity of Lord's but does have the large, sweeping OCS stand which would make a natural baseball stadium, and a more progressive ownership. It would be the best fit in terms of the playing field, but I think MLB might want to install additional temporary seating if they used it