When Flow Simulation is not showing under 'Tools', 'Add-ins' it usually means that this add-in is not installed on the machine. This also causes the button to activate the add-in to be grayed out. You would want to modify your install to include Flow Simulation like this:
- Go to the Windows Control Panel
- Go to "Features and Program"
- Locate the SolidWorks installation in the list of program and elect to "Change" it
- Select "Modify the individual installation" and proceed next
- Do not add a serial number or change the serial number but immediately proceed ‘Next’ to the ‘Product Selection’ screen
- On the ‘Product Selection’ screen locate and select the option for ‘Select different packages or products’
- In the dialog that appears select ‘SolidWorks Premium’ and select ‘Also show products that are not part of this package then press ‘OK’
- In the list of products locate and select the toggle for ‘SolidWorks Flow Simulation’ then proceed ‘Next’
- If prompted to select a location for installation files select ‘Cancel’ and the program will then attempt to download the missing files
- At the ‘Summary’ screen select ‘Download and Modify’
Once you have allowed the installation to modify the add-in should be listed and the button no longer gray.