The link in the Source SDK is actually out dated as Softimage has made a new version 4.2 of their tool and no longer support for exporting SMD from the old one.
You can find XSI 4.2 at Fileshack and Fileplanet.
The new XSI is targeted at gaming in general and not just the Source engine there for a plug in must be downloaded, you will also find that on Fileshack and Fileplanet.

To install the exporter plug-in open XSI and select Install form the file->Add-On menu, to and select the valvesource.1221.01.xsiaddon file that you downloaded, then press install. When its done you will need to restart XSI.
Now go to Softimages web page and registre to get your activation key by e-mail.

To ready your model for exporting all models must be polygon object andl need to have a texture projection set up and then have a material with a texture as the defuse color assignee to it.
If you have any NURBS object select them and then convert them to poly by selecting Nurbs To Mesh under Poly. Mesh in the Create section
To set up a default texture projection just select a method from the list under propertys-texture projection in the Get section.
I suggest using a OGL combined material, tho any material with a texture as the defuse color should work.
To change the texture used by the OGLcom material, open it and press the edit button and then the ... browse button and select a texture.

Next go to the ValveScource menu and select Export SMD, then fill in your e-mail the activation key.
Select model "Model SMD" as file type and hit Ok.