Alien Vendetta MAP18 - Lake Poison

[Gummo.nz]

I am stuck on this map. All I have done is killed all the demons in the lake at the very start and now I don't know where to go. When I go up the stairs to the building I can:
1) Turn left and go through one door before coming to a dead end (I need key cards to advance).
2) Turn right and come up against a door that won't open. I did find a secret behind a computer but I haven't been able to progress any further.

Also is there a walkthru for Alien Vendetta in case I get stuck again?

EDIT:
I was just trying to play Alien Doom and I installed it the same way as Alien Vendetta, that is I placed the files in Snowberry's addon folder and then under Ultimate Doom in Snowberry I ticked the WAD file. I can play Alien Doom but all the Aliens and objects are incorrect, eg. dead cyberdemons lying all over the place. I also tried ticking the dehacked patches as well but it has the same result.

How do I go about this?

[Grazza]

Also is there a walkthru for Alien Vendetta in case I get stuck again?There are plenty of demos. But note that there were two releases of Alien Vendetta, and most of the demos will only play back on the version of the wad on which they were recorded (I say "most" rather than "all" because some maps were unchanged in the second release).

Links:
Demos on the initial version:
http://www.doomworld.com/sda/av.htm

Demos on the final version:
http://competn.doom2.net/pub/compet-n/pwads/av/ (divided up into categories
http://competn.doom2.net/incoming/compet-n/pwads/ (the ones starting with "a")
http://www.doomworld.com/sda/av2.htm

[Gummo.nz]

Thanks,

But with regard to Alien Doom I downloaded it from here:
http://gzdoom.doomwadstation.com/class.html#jivesnew

I then placed AlienTc.wad, adcud19.bex & adcud19.deh (I wasn't sure which one Doomsday would use) into Snowberry's addon folder. The AlienTc.wad & adcud19.deh show up in Snowberry so I tick both of them and then play.
Unfortunately the weapons & aliens look like normal doom stuff. What am I doing wrong here?

EDIT:
By the way, say I just want to play a single map how do I go about changing the difficulty level? To use map 18 as an example, I tried starting a new game on Ultra-Violent then typing in the console: "setmap poison_lake 18" This took me to map 18 but changed the difficulty level to something easier.

[Gummo.nz]

Just a quick word:
1) I ended up using gzdoom for Alien Doom.

2) I started an alien vendetta game on ultra-violent and then skipped the first seventeen levels using the console command.