I'm trying to add 'other subset constraints' into the properties of a table and I do not know how or in which line; validation rules etc.:

they're like: Column C1 can only have a value if column C2 has value <value>. this column C2 is set to choose between two values.

Can somebody help please ..

Jet does not support validation rules that are dependant on values of other columns. You would need to apply these rules at the form level. Or, use a RDBMS that supports triggers as the be database rather than Jet.