Script to hide/show field based on Subject chosen

I have bee trying to get a code that will show a lookup field when a particular Subject is chosen. The code hides the field no matter what is chosen. Any advice as to the syntax on what to use would be appreciated. Below is what i've been using.

// If "Case Category" (subjectid) equals Catalog, display the Catalog (fbsm_catalogid) field.  Otherwise, do not display this field.
if (crmForm.all.subjectid.SelectedText == 'Catalog')
{
crmForm.all.fbsm_catalogid_c.style.display = "block";
crmForm.all.fbsm_catalogid_d.style.display = "block";
}
else
{
crmForm.all.fbsm_catalogid_c.style.display = "none";
crmForm.all.fbsm_catalogid_d.style.display = "none";
}