PageLESS has been updated to be compatible with DMS, but if you aren't using the latest version, it will not work.
You definitely do not need to re-purchase, but you do need to update to the latest version. You should be able to install the plugin without paying through the built in store system within DMS. If not, you could try downloading through the shop: www.pagelines.com/shop/plugins/page-less. Since you've already purchased, you should not be prompted again to purchase as long as you're logged in.
Once you download the latest, the easiest thing to do would probably just delete the old version, and install the latest as you would normally with a plugin. In the future these products will be auto updated through DMS but I don't think that system is in effect yet.
If you continue to be prompted to pay for the product again, I would contact PageLines about this, as you should not need to.