Network shares aren't mapped to the filesystem (like /mnt or /media) any longer and go through a userspace plugin (like FUSE) which is only available to the file manager and native Linux applications. Crossover needs a filesystem mount in order to access the file since it can't go through userspace filesystems.

In a terminal run these commands, changing to match your system.

$ mount.cifs //(SMB IP address)/(Share name) /media/(Mount point name) -o credentials=/root/.servercred,rw

.servercred contains the following:

username=DOMAIN\username
password=(whatever your password is)
domain=DOMAIN

Alternatively, the following added to /etc/fstab works:

//(Server IP)/(Share Name) /media/(Mount Point) cifs credentials=/root/.servercred,iocharset=utf8,file_mode=0777,dir_mode=07770