You need to indicate in what State you reside (actually where you worked).
Send your former employer a letter by certifed return reciept mail. Restate the facts of the agreement and demand that they pay you within 10 days. After 15 days or so contact an attorney. You may also want to file a claim with your State's Dept. of Labor.
Many States hold employers, who illegally withhold wages, responsible for the ex-employee's attorney fees.
You should schedule a formal consultation with a lawyer. A good means of obtaining an attorney referral is by contacting your State's bar association or your local county bar association. They probably have a web site and charge a nominal fee for the referral. Most attorneys either provide a free initial consultation, or charge a nominal fee.
Good Luck, Elliot Jay Goldstein (offices in Tampa and St. Petersburg)
The above information is provided as a courtesy, without any consideration and without knowledge of the specific facts. Do not rely on this information. Seek a formal consultation with an attorney.