You should see the little Merch Wizard icon at the top right-hand side of your Chrome browser window, circled here in green:
If you don't see the icon, there are a few reasons for this.
It's possible to hide the icons, even though they are installed, by hovering to the right-hand side of the address/search bar in Chrome and click/dragging to resize it. It could be that you have more extensions installed than there is space to display them, so you need to click the three dots to the top right of your browser window. Any extensions that aren't visible will appear as icons on the first row of the popup window. See how Merch Wizard might appear on another row:
If you want your Merch Wizard icon to appear somewhere other than where it is, you can click-hold-drag it to reposition it somewhere else in your row of other Chrome extensions.
It's worth checking you have the extension installed in the first place. Click chrome://extensions
to open this link in a Chrome if it's your default browser, otherwise just copy/paste it into your Chrome address bar and hit enter.
Then search for 'Merch Wizard' and you should have a screen like this
If this is the first time you have installed Merch Wizard, jump to step 2