At the top of the window, you should see a button (in between Play & View), after you click on it, you can click on Output Settings. A window will pop up, and if you scroll down a bit you can see File Settings. From there you can change the format that the animation will be produced in (I think its automatically set to .tif) and also choose where it's saved in.
I think it'll automatically save it into your outputs folder, which you can find by going into your C: drive, Opentoonz stuff, and finding it, but I find it a lot easier to click the "..." button and change it so it'll export to your Desktop or somewhere that's easier to access.
As for it being exported in .tif, you can change that format by clicking on the little arrow next to the box and choosing a different format. Opentoonz normally doesn't have the mp4 format, so if you want to use that you can either try exporting and then converting it, or putting ffmpeg into Opentoonz.
Hope this addresses your problem and helps! ; v ;