Where to open Inventory Period?
Inventory-->Accounting Close Cycle-->Inventory Accounting Periods

Where to enter quantity for an item?
Create miscellaneous transaction of type 'Account alias receipt' using Inventory-->Transactions--> Miscellaneous transaction

JDeveloper Version for R12.1.x

Find the correct version of JDeveloper

In order to find the correct version of JDeveloper to use with eBusiness Suite 11i or Release 12.x,

you should refer to metalink note with ID 416708.1

For example, for Release 12.1 : 

ATG Release 12.1 Version	JDeveloper 10g Patch
12.1 (Controlled Release - only included for completeness)	Patch 7315332 10G Jdev with OA Extension ARU for R12.1 (Controlled Release)
12.1.1 (rapidInstall or patch 7303030)	Patch 8431482 10G Jdeveloper with OA Extension ARU for R12.1.1
12.1.2 (patch 7303033 or patch 7651091)	Patch 9172975 10G JDEVELOPER WITH OA EXTENSION ARU FOR R12.1.2
12.1.3 (patch 9239090 or patch 8919491)	Patch 9879989 10G JDEVELOPER WITH OA EXTENSION ARU FOR R12.1.3
12.1.3.1 (patch 11894708)	Patch 9879989 10G JDEVELOPER WITH OA EXTENSION ARU FOR R12.1.3