How to view source of Google Chrome extensions?

Is viewing plugin source code possible?

Yes.

Both Minimalist for Gmail and Minimalist for Google Reader are open source projects with GPLv3 licenses. You can browser their source code trees, download the full source code repositories and more at their respective Google Code home pages:

Minimalist for Gmail

Minimalist for Google Reader

Does someone know what exactly are they accessing? Or is that there more pro forma?

Slightly tongue-in-cheek: yes, the developers know what they're accessing. Which is really to say, you should ask them in their forums. Or read the source code. I'm sure they'll happily discuss their data access routines with you. Open source software also has lots of open forum discussion around it and that's A Good Thing(tm).

Another option is to download the CRX file (right-click the Install button and select Save-As), then open it in a ZIP compatible archiver like 7-Zip.

The extension should have a manifest file which is essentially just a JavaScript text file. The extension's code itself is also a JavaScript file which you can view.

Some extensions don't do any work locally and simply pass off the work to an external server to do which of course means you cannot see the code or what happens to your data there. However for simpler extensions, you can see the code and vet it yourself before installing (assuming a decent knowledge of JavaScript of course).