Quickest way: Associate it from SPD, the option should be available in the ribbon when you have selected your workflow in SPD. Probably it will show you some options (Lists I suppose) and you will immediately see if it is possible to associate your workflow to that list or not.
In case the workflow cannot be associated to the list you want to associate it to, this can be because of several reasons:
I'm not familiar with 2013 workflows, but the gist of Reusable Workflows is: Reusable workflows are usually either Site(?) or Content Type workflows (List Workflows are not reusable), as they need to be "Reusable", i.e. they use properties that are defined in a common place and can be reused, which is why the workflow is also using this definition, which is what makes the workflow itself reusable. Content Types are reusable (you can create lists based on this content type) so Content Type workflows are also reusable. So looking in a list to associate the workflow is usually not enough, because the criteria for the workflow to show up there depend on how the reusable workflow was created.
To figure it out you can ask yourself the following questions:
What type is the reusable workflow (2010/ 2013) ? In case of 2010, what content type did you associate it with ? Does the list you want to associate the workflow to contain said Content Type ? When checking the List/Library to associate the workflow, did you make sure to select the correct content type first before looking in the list of possible workflows ?