This instuction is created on Eclipse Galileo and could be slightly different on another version of Eclipse.
Step 1 - Import WAR file
Select File -> Import
Select Web -> WAR file

Enter the name of the WAR file

Clicks Finish to import the code into your Eclipse workspace.
Step 2 - creates server
Clicks File -> New -> Other
Selects Server -> Server

Clicks Next

Selects Apache -> Tomcat v6.0 Sever. Note: You can use Tomcat v5.0 or v5.5 if this is your perference.
Clicks Next

If you already have Tomcat installed on your machine, clicks Browse and select its installation directory.
Otherwise, clicks Download and Install and follow the flow to install Apache Tomcat on your local machine.

Clicks Finish
Step 3 - add server to project
Selects Project -> Properties
On the left panel, clicks Java Build Path
On the tab panel, selects Libraries

Clicks Add Library

Selects Server Runtime and clicks Next.

Selects Apache Tomcat v6.0 and clicks Finish.
Step 4 - selects Java Compiler
On the left panel, selects Java Compiler.

Under Java Compliance, ensure you are using Compiler compliance level 1.6 as selected in the example.
Clicks Apply and clicks Yes to build the project.
On the left panel, selects Project Facets

Under Java, ensure version 6.0 is selected.
Clicks Apply
Clicks OK to close the Properties panel.