Robin,
The TPS54120 PSpice model is encrypted and cannot be imported into SystemVision. In order to import the model into SystemVision, you would need to obtain the unencrypted PSpice model. You will need to work with your TI representative or distributor to request the unencrypted model. An NDA may be required to obtain the unencrypted model as well.
You may also contact the Product Information Center at (972) 644-5580 or by filling out the form at:
http://www-k.ext.ti.com/sc/technical-support/email-tech-support.asp?AAP
Britt