If Bunk58’s last suggestion didn’t work, let’s back up and make sure your XAMPP install is working.
Normally, you’d have xampp/htdocs/modx
Is that what you have?
If so,
1. Put a file called hello.html in that modx directory, with just <h1>HELLO!!!</h1> as content.
2. Rename .htaccess to ht.access
3. Point your browser at
http://localhost/modx/hello.html
If you get the "hello" message, we’ll be sure that your XAMPP install is working and set up normally.
If that works, try Bunk58’s direct access to the manager again. Leave the ht.access file renamed. You shouldn’t need any .htaccess file at this point (and it doesn’t affect the manager anyway).
BTW, this just occurred to me, make sure the .htaccess you modified earlier isn’t the one in the Manager subdirectory. It should just be turning off the rewrite engine and shouldn’t ever need to be modified.