Have you first created the new project with particle project create?
To add a v2.0 library to a project, this project needs to be v2.0 too.
// ---------- without a v2.0 project ----------
C:\tmp\test>particle library add internetbutton
Project or library not found in directory C:\tmp\test
// ---------- with a v2.0 project ----------
C:\tmp\test>particle project create
What would you like to call your project? [myproject]:
Would you like to create your project in the default project directory? [Y/n]: n
Would you like to create your project in C:\tmp\test? Type "n" to cancel. [Y/n]:
Initializing project in directory C:\tmp\test\myproject...
> A new project has been initialized in directory C:\tmp\test\myproject
C:\tmp\test>cd myproject
C:\tmp\test\myproject>particle library add internetbutton
> Library InternetButton 0.1.11 has been added to the project.
> To get started using this library, run particle library view InternetButton to view the library documentation and sources.
C:\tmp\test\myproject>