Hi Tehmina,
Code assets in the offer library can only be used with the Form composer. They cannot be accessed via the Visual Experience Composer (VEC). If you want to use this code in the VEC you should paste your code into the custom code section of the experience.
If you have thoughts on how we should incorporate the offer library into the VEC in a future release, please submit them via a client care ticket.
**updating my response**
You can use the offer library with the Visual Experience Composer using the Swap Offer functionality. Swap Offer is really intended for you to replace content on the page, not to load additional js libraries, though. If you wanted to attempt this, you need to make sure your library is wrapped in a <script></script> tag, as offers are expected to be HTML code. Again, this is a non-standard use case and probably beyond what we can answer through the forums.
Daniel