User access to DESelect is managed by the standard Marketing Cloud Installed Package licensing functionality. Once a user has been given access to the DESelect Installed Package, (s)he can open DESelect via the AppExchange menu. No additional setup is required in DESelect.
- Go to the DESelect Installed Package via Setup > Platform Tools > Apps > DESelect
- Click on the Access tab to enable/disable access per user/business unit.
For more details, please check the Salesforce documentation on assigning users access to Installed Packages.