So to be able to explain this to you, gotta explain the phases in Magic. To do so, here is an image taken from MTGO: http://i.imgur.com/gHAq9r6.png. The following are the phases in Magic:

Untap, Upkeep, Draw, Pre-combat Main Phase, Combat Phase, Post-Combat Main Phase, End Step, Cleanup Step.

The Combat Phase is comprised of various sub-steps such as Begin Combat, Declare Attackers (labeled as 'Attack' on MTGO), Declare Blockers (labeled as 'Block' on MTGO), Damage, then End Combat. Each of these sub-steps are separated by the yellow line in my example image.

The first opportunity your opponent will have to use Cryptic Command to tap down your blockers is going to be in your Main Phase. In other situations, you'll see the active player go "Move to Combat?" as an indicator that they're wanting to move from their main phase directly to declare attackers. This is the point you need to cast your Cryptic in order to tap down their team. At this point, you're still in the begin combat phase before declare attackers have been made. If you have a manland (such as Celestial Colonnade, etc.) you'll have to activate them before going into declare attackers phase because if they're not activated before that phase, they are not eligible attackers.

Does that help?

TLDR; If somebody is going to cast Cryptic to tap down your team, you need to do it in the Pre-Combat Main phase or in Move to Combat/Begin Combat phase. This is the active player is able to even declare attackers from those eligible.