Ideally, you should ask your web host. If they tell you to rename files to a .php5 extension, tell them that you want the .htaccess alternative. On a host that has PHP 5, one of the following .htaccess files should enable PHP 5:
AddType x-mapp-php5 .php
or
AddHandler application/x-httpd-php5 .php
or
AddHandler cgi-php5 .php
Place the .htaccess in the directory above where you want to install WSN -- the parent directory. If installing WSN at /public_html/, place the .htaccess at /.
There's no way to know which type of host you have except to try all three options until one of them works -- or just ask your host which to use.
If you're looking to upgrade an existing WSN installation in which you're using URL rewriting, and thus already have an .htaccess file, you can either make that the .htaccess file for its parent directory or combine it into WSN's .htaccess by entering it at Admin Panel -> Settings -> SEO -> Other .htaccess content. Don't simply edit the file manually, as it will be overwritten regularly.
If this method doesn't enable PHP 5 on your web host, you'll need to communicate with your web host.