Normally I'd say:
Your best bet is to wait for the next beta
but what the heck,right?

Add a file called contagion_clasp.txt to your res/cardfolder and put the following in that file:
- Code: Select all
Name:Contagion Clasp
ManaCost:2
Types:Artifact
Text:no text
T:Mode$ ChangesZone | Origin$ Any | Destination$ Battlefield | ValidCard$ Card.Self | Execute$ TrigPutCounter | TriggerDescription$ When CARDNAME enters the battlefield, put a -1/-1 counter on target creature.
A:AB$Proliferate | Cost$ 4 T | SpellDescription$ Proliferate.
SVar:TrigPutCounter:AB$PutCounter | Cost$ 0 | Tgt$ TgtC | CounterType$M1M1 | CounterNum$ 1 | IsCurse$ True
SVar:Rarity:Uncommon
SVar:Picture:http://www.wizards.com/global/images/magic/general/contagion_clasp.jpg
End
That should add a working
Contagion Clasp to your Forge. However, now you have to do something for me!

Whenever you experience problems/strangeness with any trigger abilites from a Scars of Mirrodin card, report it right away, because those are the only ones currently using the Trigger system.