You can use AppSearch table with Signature and DrLocator tables to locate the path of a file. Refer to IS KB article Q103147.
You can also use AppSearch table with Signature and RegLocator tables to locate the path of Notes executable from (App Paths, if there is one there).
Once you have the path to the Notes executable in the property from AppSearch table, you can use it in Environment table.
Sorry for the high level explanation, I am on a vacation, I don't have ISAS at home, I can't get too detailed.