the box is givent to you as you do some of the early season quest, you should have a box in your inventory name Leveling aid box

If you dont have it in your inventory maybe you just put it into the storage Or if you used the timepiece - look on the old char that was season maybe the box is there

Also remember that you may not have opened one of them along the way so you may not be looking for a lvl 61 box but a lower level one. You can search through every toon in the disconnect window (minus the one you're on) by clicking any other toon, looking at their inventory and then using the search button found above the inventory. It will search every other toon and tell you the name along with it being clickable to jump to that toon's inventory. "Aid Box" should be sufficient. This box isn't case sensitive but some in BDO are.