I'm currently running Gentoo 0.5 with kernel 2.6.11.
Emerged ndiswrapper and installed ndiswrapper 0.12-r2.
Got the working bcmw15.inf and bcmw15.sys files from a friend with a exact
laptop as mine, which is the HP nx9105. He as well runns on Gentoo with same kernel but not the same version on ndiswrapper.

When i run as root:
# ndiswrapper -i bcmw15.inf
# ndiswrapper -l
hardware present bla
# modprobe ndiswrapper
FATAL: Error inserting ndiswrapper (/lib/modules/2.5.11-gentoo-r6/misc/ndiswrapper.ko): Unknow symbol in module, or unknown parameter (see dmesg)
#dmesg
ndiswrapper: Unknown symbol wireless_send_event

It works perfectly for my friend.. Tried to install an older version of ndiswrapper but it fails.

Please, really need to have my WLAN working.

Can I ask why you are trying to use an obsolete pre-release version? At very least move to .12 if not the latest release version. If the latest release still gives you that error, try dropping back to 0.7 as somewhere around .12, the ndiswrapper group seems to have made some changes to accomodate the 2.6.10 kernel and that caused trouble for earlier kernel versions.