When you use a standard Linux package (such as RedHat, SuSe, or Debian), your kernel (and/or kernel modules) would already be configured by that package, but when you compile your own kernel - as you do when installing MOSIX, you need to make sure that the kernel configuration suits your hardware and contains all the necessary device-drivers and file-systems that you are using.
One tool that often helps in constructing the correct kernel configuration is to use the output of "gzip -cd < /proc/config.gz", produced on the originally-supplied kernel, as a basis for the new configuration (but note that not every Linux distribution has "/proc/config.gz"). This output may not be totally accurate because it comes from a different (usually older) Linux kernel-version, but is a good place to start: place it in the file ".config" of the kernel-source directory, then adjust it by running "make menuconfig".
Another tip that may help to configure the kernel correctly, is that
unless you are a very experienced Linux system-administrator, you should
probably avoid the "initrd" hassles and configure all the drivers and
file-systems that you need in order to get the system to start within
the kernel itself rather than as kernel modules.