You can use the SECURITY function to do this.

eg. Let's say you have an S&P 500 futures contract already open.

The formula you would use is:
C-SECURITY("C: rading Data\Futures\Cash\$SP",C);

Substitute the path/symbol appropriate to your data.

Note that MetaStock has no way of knowing a relationship between a futures contract and its underlying cash counterpart since the MetaStock data format treats all securities independently in most cases except for composite securities where you define the relationship between two securities.

To create yourself a composite security using the Downloader. File -> New -> Composite Security and follow the instructions from there.

You should also consider the effect of:
a) Dividends (they are already factored into the futures price but most indices are unaffected by cash dividends - i.e. they will drop when a constituent pays out a dividend)
b) Interest rate/holding costs.

Search on "futures fair value" for more information about how to compare futures with their underlying.