If you're saying you have a value that you know to be in one column of a collection and you want to retrieve a value from another column in the same row, then the easiest way is with Utility - Collection Manipulation::Filter Collection. Assuming that the value you're looking up is unique, it will return a single row. You just store that result into a new collection that will then only have one row and you just need to use a calculation stage after that to get the value from the other column like [collectionname.otherfieldname] and store it wherever, etc.
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Dave Morris
Cano Ai
Atlanta, GA
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Dave Morris, 3Ci at Southern Company