In Eclipse, select Help > Install New Software… In the dialog that appears, click on the Add… button. Enter the update site URL into the «Location» text box: http://update.architexa.com/4.2/client.
Leave the name text box empty (the name will be retrieved from the update site). Click OK.
The main pane should update to show the Architexa RSE Client. Select the checkbox next to it. Click Next.
Review the features that you are about to install. Click Next. Accept the license agreements, click Finish, and restart Eclipse when prompted.