When using the NuGet Package Manager in Visual Studio, you will get files installed to a packages folder in your source tree. In Accelerator-generated systems we include a nuget.config file at the top of the source tree which contains this configuration:

<configuration>
  <config>
    <add key="repositoryPath" value=".\NuGetPackages" />
  </config>
</configuration>

This tells NuGet to install the packages for all projects into the top-level NuGetPackages folder.

The packages will almost always include dll files, and will often include exe files too. Version Control systems, like Git and TFS, will usually ignore these file types by default. Whenever you add a new NuGet package, you have to make sure you manually add all of the package's files to source control.

Git (TortoiseGit):

Right-click on the NuGetPackages folder and find the TortoiseGit menu option "Add...". In the dialog that appears, check the "Include ignored files" checkbox at the bottom to see the dlls, exes, and other ignored file types. Select them all and commit.

TFS:

Open the Source Control Explorer, and expand to see the NuGetPackages folder. Right-click and choose "Add items to folder". In the dialog select all of the package folders and click the Next button. The next dialog has two tabs: Items to Add and Excluded Items. Open the Excluded Items tab, select everything, and click on the Include Items text in the lower right of the tab. This will add all of the files to your Pending Changes list so you can check them in.