Installation

Unfortunately, SAP licensing restrictions do not permit redistribution of the SAP Java Connector (JCo). This makes the installation process a bit more complex than for the usual Eclipse extension. You'll have to perform three steps to get a working installation of the SAP R/3 Connector:

Download and install the Import Wizard.
Generate the JCo bundles and fragments.
Download and install the remaining parts of the connector SDK.

Note that these instructions assume that you have already downloaded the JCo archives (registration required).

Requirements

You'll need a recent version of Eclipse 3.5 to install and use the SDK. The SDK requires a lot of modeling components, so if you have the choice, grab the Modeling package. The modeling components are development-time dependencies only; applications developed with the Connector SDK can be distributed without them.

Step 1: Installation of the Import Wizard

Add http://rcer.sf.net/update-site to the list of update sites: Select Help - Software Updates, switch to the Available Software tab, press Add Site and add the location.
Expand the update site entry and the category and select Import Wizard for SAP Java Connector. Important: Do NOT select the SDK yet - you won't be able to install it because of missing dependencies.
Perform the installation and restart the Eclipse Workbench.

Step 2: Generating the JCo Bundles and Fragments

In your Eclipse Workbench, select File - Import....
From the list of import sources, select SAP R/3 Connector - Create SAP Java Connector Plug-ins and choose Next.
If you haven't done so yet, download the JCo archives for the platforms you want to support (registration required). Note that the Connector was designed to use JCo versions 3 and up - it will not work with versions prior to 3.0.0.
Select the locations of the downloaded archives. Obviously, you need to specify at least one file before continuing.
On the last page, you can choose whether you want to create the plug-in and fragment projects for the platforms you have selected archive files for. If the projects already exist in your workspace, you'll be prompted whether you want to replace the existing projects instead.

You can also select whether you want to export the generated plug-ins to the dropins folder of your Eclipse installation automatically. If you deselect this option, you'll have to install the plug-ins manually before you can install the SDK. Please provide feedback if the export to the dropins folder works - this is more or less experimental coding with a lot of guesswork regarding the location of the dropins folder.

Select Finish. This will generate the projects and export them if you have selected the corresponding option. This might take some time.
Restart the Eclipse Workbench.

Step 3: Installation of the Connector SDK

Select Help - Software Updates, switch to the Available Software tab.
Expand the update site entry added when installing the wizard and the category below. Select SAP R/3 Connector SDK.
Perform the installation and restart the Eclipse Workbench.

Zipped Distribution

You can always grab a zip file containing the latest features and plug-ins from http://rcer.sf.net/rcer-current.zip. This can be handy for automated build processes.