I have 2 lines coming from the NID to a whole house splitter then cat5 from there to each jack.
The NID to splitter has 6 wires (3 pair - blue/blue-white, green/green-white, orange/orange-white).

At the NID the blue pair is Line 1 (phone) & orange pair is Line 2 (DSL).

The cat5 to wall jack has eight wires (adding brown/brown-white)

The PC wall jack has 4 screw connectors & previously had voice phone but now only DSL. I want to use a Radio Shack 2-outlet modular adapter (#279-357) at the wall jack to use both the phone & DSL.

How should I attach the 8? Wires to the wall jack so the adapter will split voice & DSL so I can use a DSL modem & voice phone off that wall jack?

What you need is a 2-jack outlet where you can individually wire each jack. You want to have voice on the first pair of the 4-pair Cat 5 cable (blue/white pair) and DSL on the 2nd pait (orange/white). At the outlet you would then attach the blue/white pair to one jack (the terminals on the jack should have red/green/yellow/black colors - connect to the red & green), and the orange/white pair to the second jack (again, red and green terminals). This way you havbe two jacks, one of which has its red/green connected to line 1 (voice) and the other has its red/green connected to line 2 (DSL).

On 2nd thought... why are you bothering? The whole point of DSL is that both voice and data run on the same pair. All you really need is a splitter and the appropriate DSL filter so that you can connect both a voice phone and your DSL modem to the same pair.

Wire it to the TIA568A spec on the jack.

TIA/EIA-568-B - Wikipedia, the free encyclopedia

PS: The whole house splitter is the way to go.

jalbertini, a double jack is THE way to go.
ebaines way will work well.
When I use a DSL splitter at the d mark, I use the brown pair.
I use blue(1st pair) and orange(2nd) pair for phone or a station phone in a system, green, the third pair, for Fax or another line, Brown pair(4th), I use for DSL, and use Brown inserts(leviton Quick port, to designate DSL.
I hate Filters Hanging out every where also.