The MACD indicator that you get with MetaTrader plots the main MACD line as a histogram and the signal line as a...well....line.
Why they have made it like that is beyond me. Some may see it as a deliberate attempt to confuse the newcomer.
And if you download the OsMA indicator (which is supposed to show the Histogram) you should change it so that all MA used in the calculations are EXPONENTIAL.
Alternatively, you can unzip the attached file to your MetaTrader/Experts/Indicators folder, then load it up to get a complete MACD with Histogram.
MACD_Complete.zip (3.13 KB)