This should not be the case, as we use a single sign-on from your DesignSpark account to provide 3D CAD download rights within DSM.
If you see a message: 'You must be logged in to access the site', firstly make sure you have the latest version from our website: https://www.rs-online.com/designspark/mechanical-download-and-installation
Then, there are one of these 2 steps you can take:
1. Go to Help/Resources tab-> About DSM -> Update your DesignSpark registration details
Login to your DS account again to refresh the activation token. This will restore full access to the library.
2. Open a Windows Command terminal (CMD.exe) as administrator and switch to the directory where DSM is installed. This line should suffice most users choosing the default install path (unless your OS drive letter is different or you have a 32-bit edition DSM installed):
cd C:\Program Files\DesignSpark\DesignSpark Mechanical 4.0
Press enter to run the command and then enter the below line.