Whonoze:
The Readium Chrome extension should have opened when you downloaded it. If you are having trouble finding it, go to chrome://extensions/ to confirm that it is installed. This will show you all your extensions. If it is installed, you can find instructions on how to launch it or any app here: _webstore/answer/1054233?hl=en
You can open a new tab in Chrome and it should show you all your installed Chrome extensions. Find Readium, click on it, and it should open.
You can also set up Chrome to show all the extensions when you launch Chrome. Click on the button with three horizontal bars in the upper right-hand corner of your Chrome window; this will open a drop-down menu of options. Click on Settings. Under Settings, look at what radial button is selected under On Startup. If Open the New Tab Page is selected, you will always see a list of your Chrome extensions when you launch Chrome.