See how to enable user defined workflows in a Web Application in SharePoint with this SharePoint Tutorial for administrators.
Usually this option is enabled but might be turned off by a SharePoint administrator.
If you want to add a SharePoint Designer created workflow to a list you must be a list administrator which means you have "Manage List" or "Web Designer" permissions.
SharePoint Designer workflows run using the system account. Instead of having full rights to SharePoint like Visual Studio developed workflows, SharePoint Designer created workflows have trimmed down rights. The workflow runs with the rights of the user who started it.
Open Central Administration and go to Application Management where you can select to manage your web applications. Select your Web Application and click the General Settings tab to open the workflow settings.
User defined workflows are created by SharePoint Designer (or a different 3rd party tool) allowing you to create new workflows using existing building blocks.
Task notifications allow you to notify people who don't have access to the site.
If you work with an internal user on a document the task is assigned to the internal user and an e-mail is sent with a link to the task. If the user clicks the link he or she can request access which has to be granted manually by a site administrator.
If you work on a document together with an external user not having access to the site or document, the task is assigned to the workflow owner. An e-mail is sent to the external user with the document attached. The external user can work with the document and complete the task by sending an e-mail back to the workflow owner.