If you've been out of college for a few years, and you feel that your employer could write you a good letter of recommendation, then definitely get one from him. Graduate school admissions committees understand that you might not've kept in touch with very many professors since your graduation, and they also realize the benefit of real-world experience, no matter which program it is that you're interested in.
As others have said, you'll most likely need a GRE, possibly a subject GRE, two or three letters of recommendation, and official transcripts from all previous undergraduate institutions. Programs vary on whether they want you to send the transcripts, or want to receive them directly from the college's administration. Be sure to check the requirements and such for each program to which you're applying.