Open MT4, go to File>Open Data Folder
, then select MQL4/5>'Experts'
and paste your MQL4/EX4 or MQL5/EX5 file in this folder.
When you reopen the platform you should now see your custom EA/Indicator in the Navigator window. You can drag and drop it to the chart(s) of your choice.
You can find custom EAs and Indicators on the MQL5 website: https://www.mql5.com/en/code/mt4