The Threshold Expression (Custom) field is based on the column name in the grid control. The Display rules dialog will show the actual names of the columns that can be used in the expression. Column Names are case sensitive, so you need to make sure the exact column name is used in the expression.
Use the following examples to understand how to format the expression:
LIKE, IS NULL
Count > 1
Strings (Use single quotes around string value)
Metric = ?Some String?
Long Column Names (use square brackets  around column names)
[Application Name] = ?EXCH?
[Knowledge Script] like ?AMAdmin%?
Date > ?01/01/2005? and Date < ?02/01/2005?
Count > 1 AND Count < 5
Count =1 OR Count > 5