Your Publisher ID can be located by logging into the user interface at http://hub.skimlinks.com/ and clicking on the "Account" on the top menu.
The Publisher ID can be found under the "Sites" subheading. It is the number located before the "X." The number after the X is your Domain ID which differentiates domains from each other should you have more than one under your Skimlinks account. For example 00000X111111 0's represents the Publisher ID and the 1's represent the Domain ID.
For Skimlinks Wordpress Plugin users: Please note that you need to enter the entire SiteID where it asks for the Publisher ID. That is, you need to enter the PublisherIDXDomainID / 00000X111111