The CodeStream plugin for VS Code is currently in private beta and is being distributed as a VSIX file. If you've already downloaded the file, you can install it using the Install from VSIX... command in the Extensions view command drop-down.
Alternatively, you can use the Extensions: Install from VSIX... command in the Command Palette. You can also install using the VS Code --install-extension command line switch providing the path to the .vsix file.
code --install-extension codestream.vsix