Hmmm.... I can see why you missed the powerup Phil. The walkthrough does say where it is, it's just difficult to visualize. Let me see if these screenshots I took make it easier to find it (click on the pics to get the original size):
"Once again, enter the greenhouse and kill the Mercenary on guard. Two more Nali Healing Fruit can be found in the plant beds. Then, go to the far side of the greenhouse from the information panel. There is a short slope at the top of which is a metal door / lift controlled by a red button nearby. Don't press the button yet, instead now going to the small lifts into the inner area. Collect the box of 50 Bullets and Health Pack here then ride the lift into the antechamber above. Partake of the two clusters of 40 Tarydium Shards in this antechamber, then go through the large door into the inner room and kill any Mercenaries that attack beyond.
You will be on a crossroads of walkways with an Assault Vest in the middle. Collect the armour then look around; three exits lead into the greenhouses whilst a fourth is locked with an informative legend:
'Access Denied. Control Room authorisation must be granted to access Noork's Elbow.'
Drop on to the floor beneath the walkways then collect what you need of the Health Pack, Flare, box of 50 Bullets and can of 12 Eightballs here. Then, stand on one of the floor plates with the pink symbol on it. These plates are lifts which will take you up to another floor above.
As you reach the top, a Mercenary will attack you. Kill him then collect the Tarydim Shards he drops. Then, explore this floor to obtain various items; firstly, collect the Dispersion Pistol Powerup in the centre. Then, visit the small ofshoot chambers in turn."
These should help the next time you go through Terraniux 😀