I'm sure its fine if you have tattoos. Besides, are the tattoos going to sing? No, they're not. If they're human and they have hearts they'll pick you because of your talent and determination :)
Here are the addresses of the companies i know.
CORE CONTENTS MEDIA
Seoul, Guro-gu, Guro-dong, #1278, Guro Dusan 108-902, 152-050
CUBE Entertainment
Seoul, Gangnam-gu, Cheongdam-dong, 125-6, 3rd Floor, Cube Entertainment
DREAM TEA Entertainment
Seoul, Gangnam-gu, Yeoksam-dong, 725-1, Jin House B1, Dream Tea Entertainment
DSP Entertainment
Seoul, Seocho, Banpo-dong, 58-8, Shin Seong Building, 3rd Floor, DSP Entertainment
F&C Academy
Seoul, Mapo-gu, Donggyo-dong, 198-11, Hoseong Building, 2nd Floor, F&C Academy
Jellyfish Entertainment
(Artist name), Gangnam-gu, Shinsa-dong, 27-26, Seoul, South Korea, Jellyfish Entertainment
J.TUNES Entertainment
Seoul, Gangnam-gu, Cheongdam-dong, 91-1, 4th Floor, J.Tunes Entertainment
JYP Entertainment
Seoul, Gangnam-gu, Cheongdam-dong, 123-50, JYP Entertainment
NegaNetworks
42-7 Arts & Business Center 3F, NonHyeun-Dong, GangNam-Gu, Seoul, South Korea, 131-101
NH MEDIA
Seoul, Gangnam-gu, Nonhyeon-dong, 10-7,8 Dongnam Building, #202, 135-15 NH Media, U-KISS
OPEN WORLD Entertainment
Seoul, Gangnam-gu, Cheongdam-dong, 13-30, Open World Entertainment
PLEDIS Entertainment
Seoul, Gangnam-gu, Nonhyeon-dong, 243-17, #301, Pledis Entertainment
SM Entertainment
Seoul, Gangnam-gu, Apgujeong-2dong, 521, SM Entertainment
STAR EMPIRE Entertainment
Seoul, Mapo-gu, Hapjeong-dong, 376-28, Star Empire Entertainment
STARSHIP Entertainment
Seoul, Seocho-gu, Banpo-4dong, 577-92, Seonho Building 2nd Floor, Starship Entertainment
SWEETUNE
Seoul, Gangnam-gu, Dogok-dong, 411-7, Gaepo Building, Sweetune
TOP Media
Seoul, Gangnam-gu, Daechi-dong, 918-10, Seongyeong Building 2nd Floor, TOP Media
Y-HOO Enterprise
Seoul, Gangnam-gu, Samseong-dong, 44-10 6th Floor, Y-Hoo Enterprise
YEJEON Entertainment
Seoul, Seocho-gu, Bangbae-dong, 137-060, 1F C Bldg. YeJeon Entertainment
YG Entertainment
Seoul, Mapo-gu, Hapjeong-dong, 397-5, YG Entertainment
And this is what I know from what I read.
If you pass the first round, there's another audition you go to, from there theres another one and finally a final audition where they get to pick the winner.
Those are just from out of the blue, the information about the audition is what i read and what i heard of. I could be wrong, but i'm quite positive that's the process of the audition.