findBPK is an home method that operates on one instance of the entity bean.
ejbActivate will operate on another instance.
You can not pass value from a home method to an instantiated bean.
Say differently, home calls can not "remember" anything.
Have a look at Fat Key pattern on theserverside, it is the solution for your problem : put state in the PK that is return by the FBPK, get it from getPrimaryKey() method on EntityContext inside ejbLoad