Peppe,

You are hitting two issues here....

1) Muvizu has a limit of 65335 faces for any one model, although you will run into trouble over around 40000. When you export the Sketchup plug-in tells you how many you have. If it's over 40000 then don't bother trying to load into Muvizu. You will have to cut up the model and import it in pieces. You can then group the pieces together in Muvizu although you will not then be able to move this object.

2) There are some quite restrictive limits when loading large models into Muvizu. You need to position yourself in the middle of the "Muvizu dome" - i.e. the whole scene and then press "Q" for 1 second to move up. This will get you into the optimum position to load large objects. Although the object you are trying to load is nowhere near too big to fit in, Muvizu will still refuse to import anything that is bigger than about a large house. For your city scape you will have to chop it up into bits and import them separately. So each building in the city scape I made for this video - was imported separately.

I does get easier with practice.
I hope this helps.