Hi Diego,

I believe that is a bug, or at least a "feature-incomplete" issue, when using a List Collector (slushbucket) variable and the Condition Builder.   I've run into that before and did not find a satisfactory response.   You can try to trick it by changing the variable type to something else (another variable type that allows for using "Contains"), setting up the condition, and then changing the variable type back. It may hold on to the value of the condition you've setup. (YMMV)

There are some inconsistencies with the way GlideLists are treated depending on where you are in the system (e.g., when using as a field vs. variable, when using in Wizards/Order Guides/Catalog vs. normal forms, etc.).   This is one sore-point I've had with the platform, but overall you won't see me walking away.  

Good luck,

-Brian