If I use Deoxys-EX's "Helix Force" attack against a Pokemon with a DCE attached, does it do 30+60 damage?
If Deoxys-EX has the required Plasma Energy attached, it does 30+60 damage. The extra damage, as the card says, is 30 for each energy attached to the defending Pokemon, and DCE provides two energy.
Source: TPCi Rules Team (2013-06-20)

Relevant cards

Note: All applicable cards may not be listed below.
BW9_111
BW9_53
BWP_BW82
BWP_BW82