Log in to your Tsohost Client Area and click on 'My Products & Services' which will be on the right hand side under the green My.Tsohost Navigation panel.
Click "Manage", which is next to the hosting package that you purchased, then click the green button saying "Login To Control Panel", roughly mid-way down the screen.
You then see a list of all your domain-names or websites. Click "Manage Website" next to the relevant domain name.
Hit "Install Applications", which is available at the bottom of the screen.
WordPress will be one of the options listed, and you can install it from this screen by pressing the "Install" button.