Where is Shinsen Shinjuku station?

I see it on a lot of route maps but can't find it. Is it next to Shinjuku station?

Did you mean, do you take the Keio Line?
Keio has Keio main line(honsen) and the Keio New Line(shinsen), different station.
Keio shinsen if, is together Shinjuku Station and the station of Toei Shinjuku Line.

Shinsen Shinjuku station (K001) is the Shinjuku station of the Keio Shinsen Line, which is a relatively new line which goes onto Hatsudai, Hatagaya and Sasazuka etc According to my map the station is situated out of the south end of the main Shinjuku station, but the Toei Shinjuku line seems to feed into the Keio line, after the Shinsen Shinjuku station. Most maps seem to call it Shinjuku, which is a bit confusing. It seems to have entrances off Koshu Kaido

Station of Keio New Line is a short from the B2 floor of Lumine shinjuku shop 1 of the south exit of Shinjuku Station.
Please look at the third page of this map.
http://www.lumine.ne.jp/pdfguide/shinjuku_f.pdf
Keio New Line
http://en.wikipedia.org/wiki/Kei%C5%8D_New_Line
It is possible to go even from the stairs of side of Lumine 1.
https://www.youtube.com/watch?v=7F4Dtwbgdhg

Named "Shinjuku" stns is most difficult
to explain by text only for 1st timer. (for Japanese 1st timer,too)
Young Japanese calling "dungeon(donjon)"
named from RPG(role-playing game) most means "maze".
Shinsen Shinjuku(of Keio line) is not "next" station. (from where?)

Anyway see the bellow 3 images. (captured with directions)
1) Layout of Keio Shinjuku[新宿] stn Main line and New line[Shinsen]
http://urx.nu/dGZi
2) Keio Main line and New line (rail route)
http://urx.nu/dGZm
3) Keio rail map from Shinkuku to Sasazuka stn.(rail route wide)
http://urx.nu/dGZo
Google map [新線新宿] is so platform B5F deeper than main line floor.
Koshu Kaido[甲州街道] is wide car road on ground. (not underground)


@ haro1210
New line platform is NOT different station.
Stn numbering [KO 01(by Keio)] and [S 01(by Toei)] both.
Connected from Main line platform [3] to New line platform [4][5].
No need thru each railway's ticket gate in/out. http://urx.nu/dH3y
Deepest platform B7F of Toei Oedo line[E 27] connected as Toei's stn right.

I understand!
I had been using the Keio new line , without riding only 1-2 times in Keio main line from Shinjuku station, I did not know that it is transfer without exiting the ticket gate.