I've been without internet for ages in my house, I've been poring over the manual for days and finding little, what should I search for?
Well, you can always just start reading, whether straight through or jumping through 'See Also' links.
For ActiveInventory, you can search 'inv' in the Index and the second item that pops up is 'Inventory Item events'. It says:
You can use the player.ActiveInventory property to distinguish which item they used.
Which links directly to the ActiveInventory property.
The page I found on the subject had one = mark.
I take it you mean the Example on the ActiveInventory entry? Well, if you read what it says,
Gets/sets the character's current active inventory item. Setting it will update the mouse cursor if appropriate.
will make the inventory item iKey active
Both will tell you that the you can set
the ActiveInventory, not just getting it (which is its most common usage). Combine that with basic scripting knowledge (look up Operators if you don't know) and you'll know that you use = for setting (and a semi colon) and == for checking (combined with brackets). This is the reason we don't like script kiddies on the forums that say "Give me teh code!!! Dont point me to a supid manual entry, or RTFM!!" because they don't understand the code that's give to them.
Also, if you had run a forum search, the first result
of 'parse error' would give you your answer. Not to mention two others in the first 5.
I realize I sound condescending, but the best thing for you is just to start reading (have you seen the Scripting Tutorials?). I don't think you're a n00b, and I'm just trying to help you become better at scripting. (and I'm willing to help anytime)