New Linux kernels (2.6) does not have /proc/net/ip_conntrack, so plugins have to use nf_conntrack instead.
Created attachment 133551 [details, diff] munin-1.3.3-fw_conntrack_plugins.patch (for new kernels)
Created attachment 133552 [details, diff] munin-1.3.3-r2 ebuild patch
Added for 1.3.4, thanks. Please send it to upstream as well.