You can use the Security function to accomplish this, i.e.:

(Security("C:\MetaStock Data\Stocks\IBM",C)+Security("C:\MetaStock Data\Stocks\MSFT",C)+Security("C:\MetaStock Data\Stocks\AMZN",C)+Security("C:\MetaStock Data\Stocks\GOOG",C)+Security("C:\MetaStock Data\Stocks\CSCO",C)) / 5

There is an add-on called FIRE which is a much better tool for this than doing it through the formula language. More information can be found at:

https://www.metastock.com/products/thirdparty/?3pc-add-fire

You should be aware that constructing an index in this fashion means that it is price weighted, and that any corporate actions in any of the companies concerned (such as a 2:1 stock split in one of the constituent stocks) will cause the index to change dramatically (effectively it will halve that company's weighting).

This is why most indexes are constructed using a market cap weighting. Indeed many indexes now use a variation of this - free float market cap.

Unfortunately the "MetaStock" data format has barely changed since the 1980's (when the file format was called Computrac) and has no way of storing any additional fields such as shares on issue/market cap, so be aware of the limitations of producing such an index without market cap values.

Cheers,
Richard.