Look in upper right hand corner of your project page and find the icon that looks like this [1:1] and click it. It will center and reset the zoom of your program.
Isn't two finger zooming an Apple touch screen thing, like we do on an iPhone? So, unless you're used to doing this on your Apple tablet at home, the two finger slide technique won't work on the class Chromebook.
A few disclaimers first, I don't have a Chromebook, or any LEGO Mindstorms sets... (yet), so I can't be totally sure of what you mean or every detail I say. So, with that said, let's dance!
So, I watched a video of cool teacher Mr. Hino, downloading the Windows version of this app on his class Chromebook.
So, to understand your question further, I downloaded the same Windows version app on to my PC, and have installed/launched the program.
Assuming your app is the same as my app, I took some screen shots to point out where the Zoom In/Out stuff is.
1) So, this picture is the Lobby/Row of Robots when we start up the program.
2) We need to get to a "Project". Located in upper left hand corner. Click to pull down the menu.
3) We'll either choose to start a "New Project", or if we have one saved, "Open Project" that already exists.
4) This picture is where you're zoom-stuck.
5) Look in the upper right hand corner and you'll see some icons in a row.
6) You'll see magnifying glasses with "-" sign, "+" sign, and "1:1" zoom reset.
7) Click "1:1" Zoom Reset.
EXTRA CREDIT ANSWER:
There are keyboard combinations called shortcuts that make life hella-easier.
The most common are:
"CTRL + C" = Copy.
"CTRL + X" = Cut.
"CTRL + V" = Paste.
Don't know if your Chromebook has a mouse wheel, but...
"CTRL + Mouse wheel UP" = Zoom IN.
"CTRL + Mouse wheel DOWN" = Zoom OUT.