If you go to www.omnetpp.org, to go 'DOCUMENTATION' and then INSTALLATION GUIDE', depending on what operating system you use, you get guides. But install OMNET++ 4.5 which is the latest version.
After installing omnet++, you can then install MIXIM, OMNETMANET, found on the right hand side of the home page, under 'FEATURED MANAGEMENT'. These tools will help you for WSNs examples. One problem you will encounter when you first install omnet++, is that you may have errors when you build for the first time, keep 'BUILDING ALL' until the errors disappear, ignore the errors messages you get, continue to 'BUILD ALL', the errors will go.
If you do not build successfully, you will not be able to run any example in those packages. I will also advice you to join the omnet++ mailing group, if you have gmail account, so that you can get help and see questions asked by people and answers given, which will really help you, because there is no problem you will get that someone has not encountered before, and i assure you there should be an answer. Hope this helps!.