When you start SimaPro, make sure to open the 'Professional' database. This is the default database installed with SimaPro.
Open your project, go to the Libraries section (under Goal and Scope) and make sure that the 'Methods' library is selected.
You should then see all available methods in the Methods section (under Impact assessment).
Please note that if you open a library (instead of a project), you will not be able to see the other libraries - including the methods library