Enable DLL import in MT4

To enable DLL, check the box in the blue circle.

If DLL is enabled globally, all EA and indicators can use DLL.

Enable DLL for certain EA and indicator

DLL can be enabled for certain EA and indicator. After load the EA or indicator, in the configure window, select the tab Common, as the image below

To enable DLL, check the box in the blue circle.

If DLL is enabled, only the EA or indicator under configure can use DLL.

Is DLL dangerous?

Yes and No.

Unlike MQL that is running in a sand box which protected by MetaTrader, DLL can access all information and files on your computer. From this perspective we can say DLL is dangerous.

However, using DLL is nothing different than you download and run any software from the internet, include MetaTrader itself. Why do you dare to run MT4 terminal after you download from your broker website without any hesitation? Because your trust MetaQuote. So if you trust the vendor who provides you the EA/Indicator with DLL, DLL is safe to you.