Conditional Stylesheets Set-up: Using the Module
Assuming you are using the conditional style module the format would be

; Set the conditional stylesheets that are processed by IE.
stylesheets-conditional[lt IE 7][all][] = ie6-and-below.css
stylesheets-conditional[IE 9][all][] = ie9.css
stylesheets-conditional[IE][print][] = ie-print.css
stylesheets-conditional[(gte IE 9)|(gt IEMobile 7)|(!IE)][all][] = modern-browsers.css


And there should be no space between the conditional-stylesheet and [IE].

If that doesn't work then you should make sure that the stylesheet it is referencing can be loaded. The stylesheet lines don't show up if the stylesheet doesn't exist or cannot be opened.