The campaign modifiers window is available in the upper right set of campaign tool buttons. The GM can build a number of modifiers that can be applied quickly to dice rolls.
To apply a modifier from the list, click the plus/minus button next to the modifier name and the numerical modifier will be added to the Modifier Stack. To hide/show a modifier in the player modifiers list, click the eye icon on the right side of the modifier entry.
Otherwise, the modifier list works just like other Campaign Data Lists. Only the GM can edit the campaign modifiers list.
Ruleset Modifier Details
Some rulesets have additional modifier buttons in the campaign modifiers window that can be toggled for the next dice roll. These correspond to common modifiers in the relevant game system (i.e. Cover, Concealment, Critical, etc.).