I don't think it is possible to miss this one.
You have to press the square on three cactuar stones, (I don't think they stay lit up, but as long as you activate them) Then the ghost of the cactuar shows up. Kimari's weapon is in the south part of Thunder Plains. Follow the ghost to the fallen tower, it's on the right side and once you've found the fallen tower press the square button, Then you should be able to get his weapon. :rolling:
cactuar stone 1 - walk out of rin's travel agency then go north, just to left of you is the first stone.
cactuar stone 2 - continue north towards the save sphere, just to the left of the save sphere is another stone.
cactuar stone 3 - from the save sphere stay to the right of the screen and continue north again. you should be able to see the next stone.