I'm not a Windows user so please bear with me if any of this is inaccurate :)
Usually after a package is installed, it needs to be added to the PATH so that the executable can be located. The installer may do this for you, but usually if an environment variable (such as PATH) is modified, it will not take effect immediately and need to be refreshed. The easiest way to refresh your environment variables is to close your shell and open a new one. So give that a try.
Then, try running `svn --version`. If you get a version number (or some other output from svn itself) then it is indeed installed and accessible. If the script still can't find svn, please report back so we can take a look. If you get some message about not finding a command called `svn` then you haven't fully installed svn, and since that's not directly a Blockly problem, you might have better luck following up with a group of Windows experts if you can find a better place to ask :)
Hope that helps,
Maribeth