if it is mounted then it should be under /mnt/share or wherever you mounted it to
\\lan\etc is not a mounted location on your local filesystem
Right, of course my NAS wasn't properly mounted that's why I didn't find it. Maybe this is useful for anyone else in future.
If you want to mount a share (in my case a simple NAS on my FritzBox) from a windows system you have to create a CIFS share, if you ran a Linux system you can use NFS which is even faster. The easiest way to achieve both on Android is using Mount Manager, it pays off to get the pro version
play.google.com/store/apps/details?id=com.twimager.cifsmanager. To do this you have to be rooted and need Busybox installed, also if the dev claims BB is not neccessary anymore I haven't read from anybody getting it to work without.
Now, your Kernel has to support CIFS (resp. NFS, but this is about CIFS now). If you ran a Custom ROM chances are your Kernel is already supporting it (i. e. CM, SlimBean, Carbon), in which case you'll find the needed modules in \system/bin/modules. Otherwise you'll have to flash a custom kernel with CIFS support (for instance Yank- Kernel).
If kernel and modules are sorted you can install MM.
1. Open MM and grant permanent superuser access.
2. Open the settings (little gears icon)
3. Check with 'Loaded Modules' if your system recognises the modules.
4. Tick 'Load existing CIFS' and 'Enable auto mounts', then scroll down and check if modules are already loaded. If not click Manage Modules'. Add first 'dns_resolver.ko', then 'md4.ko', then cifs.ko. Press back.
5. Now tick 'Load Modules' and 'Load on boot', make sure that when you click 'force loading of modules' it says 'CIFS modules are loaded'. If you want tick 'Auto Mount on Boot' and same with wifi, then go back.
6. Click the +icon to create a new mount.
7. In 'share path' you must enter the path to your CIFS share in this format //192.xxx.x.xx/music
8. Now you enter your desired mount point. I got an empty directory in my mount if I didn't use exactly this path: /data/media/0/cifs/music
9. Choose Mount Type 'CIFS' and tick 'Automount'.
10. (+) Add the follwing options and enter the corresponding values:
user = your cifs username
password = your cifs password
file_mode = 0777
dir_mode = 0777
iocharset = utf8
custom (leave free on left side): noperm
Click OK and then tap once on your mount point to mount the directory.
If everything went fine you now see a green check mark. if not you get an error toast. In this case start with checking the FAQ in settings.
Your share is now mounted and you can add GMMP to scan your music directory on your NAS. It took more than 16 hours for GMMP to scan and add 250 GB of perfectly tagged mp3 files with embedded cover art in each file. But now it's perfect and playing music everywhere in the house on my S3 has never been more fun.