Prerequisites :
Assuming you have installed all the above products successfully, Just follow the steps as below :
Configuring the Office Web Apps Services : Go to Central Administration > Click on Manage Service Application. All the menus are not displayed in ribbon Menu format which was first introduced in Office 2007. Click on New > Word Viewing Services ( You can choose PowerPoint or Excel also, steps are same ). This will open a pop window.
Adding Services for Office Web Apps
Give a Proper Name which can have your companies or project name. Under Application Pool select : SharePoint Web Services Default. Next keep the check box checked which says : Add this service application’s proxy to the farm’s default proxy list. Click Ok
Adding Word Viewer as Service Application
Office Web Apps as Services in Sharepoint 2010
This will install all the Office Web App services required. You can see the name as you gave in the above step. Activate Office Web Apps in Site Collection. Once the services are ready, to enable Office Web Apps you will have to activate it to use it.
Go to the site for which you want to activate this feature. Click on Site Action > Site Settings > Site Collection Administrator > Site Collection Features Activate Office Web Apps.
Activate Office Web Apps Feature in Site Collection
How to make sure Office Web Apps is working for your site collection ?
Locate any office document you have and click on the smart menu which appears when you hover your mouse on it. Don’t double-click as this will launch the document in Office Client if its installed. This feature can be changed. If you see View or Edit in Browser as menu item, your Office Web Apps is configured correctly.
viz: http://www.technospot.net/blogs/how-to-enable-office-web-apps-on-sharepoint-2010/