Hey mate - it sounds like an o-ring has been damaged (or omitted entirely) when the sleeve was assembled (our fault, not yours). Shoot me an email (steve@vorsprungsuspension.com) and I'll have you sorted out with the appropriate seals within a day or so. Where are you in Melbourne?
The way the Corset works is that it's two sleeves, one over the other. The outer one isn't keyed to the inner one, so with enough force you can rotate it over the inner sleeve. There are several o-ring seals between the two, and one may have been pinched during installation. To get the sleeve off, you can remove a large thick o-ring at the far end from the threads (use a pick or something sharp to pry it out - it's just a retainer not a seal, so unless you fully break it, damage isn't an issue) and then slide the outer sleeve off over the inner sleeve. Be careful as it will have a bit of friction initially then release quite quickly - twist to break the stiction then gently pull at the same time. Once you've done that, you'll be able to get a hold of the inner sleeve to install or remove as needed. I'm in the process of updating the installation instructions to account for this plus the initial confusion you had over whether you needed to change seals or not.
Sorry for all the issues, but we'll get you up and running ASAP! - Steve