I want to add weight to a WBS.

What Manuel said is the way it should really be done. You want to create the project, add the WBS's, add their respective activities with duration and cost and the EVM/CPM functionality in P6 will roll the most important WBS/Activities up to the project level. If you're manually trying to do it, the software is basically just a fancy way of doing a schedule. I do understand what Tony is suggesting, but I feel like this is before the hard schedule is in Primavera and more on a narrative basis for early stakeholder meetings before the WBS/Activity/Cost/Resources are starting to really get built out. If you have cost associated to all of these (contracted) and you assign the % to each of them, a simple xls formula will give you your "weighting"...you don't need Primavera for that. Primavera is used to manage projects with cost, schedule and scope. We do allow you to have a budget log (static) on each of the WBS's and that's one way you could go with your weighting, but once the schedule starts getting built out, that static budget log is just used for comparison and is not in any impacted by the schedule.