Hello are you searching old SDK, right?
You need to replace from flex-description.xml into 4.6.0 If you are using with Flash Builder 4.6 yet than you download same Flex SDK 3.2 from here.
PS: You want to use Flex Plugin for Eclipse right. I am sorry for latest new version of Eclipse won't load old plugin because it is old.
You need to use under Flash Builder 4.6. My old Flex SDK 3.2 or 3.5 work fine under Flash Builder 4.6 before replacing version number from flex-description.xml example:
<version>3.2.0</version> to <version>4.6.0</version> Than you can use components into design mode
Thanks and happy coding!