Maybe you didn't type quite what you meant, but the If statement should look like:
If (field1 = "some value" or field2 = "some other value, ; Value if true ; Value if false )
The statement in this case should look something like:
If ( IsEmpty (MyTable::SurveyMailed) or MyTable::SurveyMailed = "Not sent" ; 1 ; 0 )
Note the values MyTable::SurveyMailed are the fields, not the values contained in them. This expression will evaluate to true if the SurveyMailed field is left empty, or set to "Not sent".
The validation can actually be shortened to just:
IsEmpty (MyTable::SurveryMailed) or MyTable::SurveryMailed = "Not sent"
Thanks for the help!