From what I gather of your question, you have an installation problem rather than a build problem. Building may be problematic if you have dependencies that are unavailable offline, but installing Scala IDE should work.
The Scala-IDE website offers a zipped update site for download for every release. The direct URL for the latest version is here.
You need to make this file available to the offline OS you're using. If you unzip it, you should be able to point to the decompressed result as a local update-site and choose the plugins you may want to install. More simply, you can also try to drag and drop that zip into the installation dialog of Eclipse.