I just tried loading Seaside into the latest Pharo 6 pre-release image from the Catalog Browser. It loads cleanly. However, the Seaside control panel doesn't start because it uses NewListModel which does not exist in this image:

WAPharoServerAdapterSpecBrowser>>#initializeWidgets

  self instantiateModels: #(
    listModel NewListModel "<== Exists In Pharo 5, not in Pharo 6"
    textModel TextModel
    toolbarModel WAServerAdapterToolbar).

Ok, how about Pharo 5? Using Pharo 50772, which is currently the latest Pharo 5 image, I loaded Seaside 3.1.5 programmatically after visually inspecting ConfigurationOfSeaside3:

Gofer it 
  smalltalkhubUser: 'Seaside' project: 'Seaside31';
  package: 'ConfigurationOfSeaside3';
  load.
(#ConfigurationOfSeaside3 asClass project version: '3.1.5') load.

The load sequence includes messages to String>>#subStrings: which is deprecated. The senders are GRStringTest>>#testSubStrings, JQAjax>>#callback:passengers:, WAAdmin class>>#register:at:in: and WAAdmin class>>#unregister:in:.

Otherwise Seaside 3.1.5 loads cleanly. Test Runner reports 1173 run, 1171 passes and 2 expected failures.