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Please follow the steps below to add your indicator on the MT4 terminal, I've also already created a short video clip for you to see how it is being done.
1.) Locate and copy the MT4 custom indicator file on your computer. These are the MT4 indicator files that you would have downloaded. The icons look like the following:
2.) Open the data folder.
3.) Open MQL4 file.
4.) Open the indicators folder.
5.) Paste the files.
6.) Restart MetaTrader 4.