I am an expert on understanding the MACD but an ignoramus when it comes to MT4.
The signal line is an exponential average of the line that you see - the MACD line.
Maybe it is just invisible and you need to click a box of some sort.
The MACD always comes with 2 lines, the MACD and the signal.
On GFT Dealbook, the program I use, the MACD has the histogram on it as well.
Some one who knows MT4 should be able to answer your question more fully.