DSLS server logs are available at below location:
C:\\ProgramData\\DassaultSystemes\\LicenseServer\\LogFiles

In order to see license usage entries (USG), its necessary to enable 'License UsageTracing' option in Server application as below:
1. Launch Dassault Systemes License Server
2. On Server definitions tab, select Computer name>Display Properties
3. In Server Configuration window, select button 'License Usage Tracing..' and select all the licenses that you want to trace.
This will add entry related to license usage in server logs.

License Usage entries in server logs appear as below:

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