Open MT4, go to File>Open Data Folder, then select MQL4>'Experts' or 'Indicators', and paste your MQL4/EX4 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.