No way to tell what's wrong until you post the code or file for people to see. Also, please explain what you expect to see in that indicator.
In response to the points made above, I would point out the following:
1. The version of MACD supplied by MetaQuotes hasn’t got two crossover lines and the histogram isn’t as responsive to price changes as the traditional version and therefore isn’t acceptable for my purposes.
2. A link to the file containing the code is here: http://www.forexfactory.com/showthread.php?t=69409
3. I have been using MACD for years and expect to see both the crossover lines and histogram responding to price and oscillating either side of the zero line.
Not sure that this is what you want but it's a starting point for you. I modified the original MACD to get close to the one you're using. I don't have MT4 so I haven't compared the two but looking at the code, the MACD line is exacly the same on both indicators and the signal line uses a different MA method which I replaced to match the one you like. The rest is cosmetics I think.
Hope it helps, regards
Thanks for your help Heighast- I added your file but the indicator still doesn't look right.