k, create a shell script with something like this:
#!/bin/bash
/usr/local/mrtg-2/bin/mrtg /var/www/html/mrtg/core/mrtg.cfg
indexmaker --output=/var/www/html/mrtg/index.html /var/www/html/mrtg/core/mrtg.cfg
I'm guessing in your browser your going to:
http://localhost/mrtg/index.html if not then change the path in the indexmaker --output= line to whatever it is.
Then run the script a few times. The first few times you run it, you'll get error messages about RATEUP and overwriting files and whatever. The 3rd or 4th time you run the script it should run successfully (no messages). Then check it in your browser and make sure you see the graphs. Then add this script file to your crontab to run every 5 or 10 minutes (or whatever).