Please follow below steps to get the dll.
1. Open the Windows Explorer
2. Navigate to C:\Windows\Microsoft.NET\assembly\GAC_MSIL\ Microsoft.SharePoint.IdentityModel
3. And you can find the dll here C:\Windows\Microsoft.NET\assembly\GAC_MSIL\ Microsoft.SharePoint.IdentityModel\v4.0_188.8.131.52__71e9bce111e9429c
But it's not so simple some time as GAC_MSIL folder is not visible (Missing) in the assembly folder.
Here are few ways you can get it :
- start > run > cmd
- type : "cd\windows\assembly"
- type: "attrib -r -h -s desktop.ini"
- type: "ren desktop.ini desktop.bak"
now open the GAC folder normally in your explorer.
Hope it helps you too.