You can use this event for when you select something from the combo box: 
https://wiki.multitheftauto.com/wiki/On ... oxAccepted 
Also, guiComboBoxGetSelected requires a combo box element to get the selected item, like this: 
function hpFunc ( ) 
    local item = guiComboBoxGetSelected ( _cmbbx ) 
    local text = guiComboBoxGetItemText ( _cmbbx, item ) 
    if ( text == "Regeneration" ) then 
        guiSetVisible ( regHP, true ) 
    end 
end