A formula is a set of requirements that must be met before someone can challenge you to a game or accept your offer for a game in the seekgraph.
To set your formula go to the toolbar above in ICC for Mac or Windows. Click on the Settings icon and choose from the list 'Formula' (Or menu above: Edit > Options > Formula)
If you want your formula to be used to screen people who accept your seek ad, you must check 'use formula in seek' (Settings > Formula)
Formulae do not apply to "rematch" challenges, and the pools like 1-minute, 5-minute, etc.
For operators and terms for formulae (when using commands) please read "help formula"