And if you want to do it manually, JMP uses C++ style comments. Either surround the code to be commented out with /* */, or start each line with //.
// Uncommented:
pi = 3.14159;
e = 2.71828;
// Commented out using /* */
/*
pi = 3.14159;
e = 2.71828;
*/
// Commented out a different way
// pi = 3.14159;
// e = 2.71828;