Your spouse can apply for the TD visa from any one of the US Consulates in China. She needs to fill DS 160 and provide the required documents as other non-immigration visa applicants do. Also she need to provdie the following documents for ther TD visa application:
The following is cited from the DOS website:
http://travel.state.gov/visa/temp/types/types_1274.html#10
Spouses and Children
Spouses and children (unmarried children under the age of 21) who are accompanying or following to join NAFTA Professionals (TN visa holders) may receive a TD visa. Applicants must demonstrate a bona fide spousal or parent-child relationship to the principal TN visa holder. Dependents do not have to be citizens of Mexico or Canada. Spouses and children cannot work while in the U.S. They are permitted to study.
- Canadian citizen spouses and children do not need visas, but review the CBP website for the port of entry requirements:
- Spouse and children are not Canadian citizens - They must get a TD nonimmigrant visa from a U.S. Embassy or Consulate. They should review U.S. Embassy or Consulate website how to apply information.
- Mexican citizen spouses and children must apply for TD nonimmigrant visas at a U.S. Embassy or Consulate,
- Spouses or children following to join must show a valid I-94, thereby providing proof that the principal TN visa holder is maintaining his/her TN visa status.
