Did you run
depmod? This is something
make modules_install does automatically for you, but if you copy a module into
/lib/modules this way, you will need to do that manually. See
man depmod for more information.
I am not sure if
depmod will report the presence of incompatible modules.
You can also use an explicit path with
insmod, which will at least test if the module can be loaded.
> insmod /lib/modules/3.0.7/kernel/drivers/net/wireless/rtlwifi/8192cu.ko
> lsmod | grep 8192cu
If the module has dependencies, they won't be loaded, which is why
modprobe make things more convenient. Note the kernel will reject modules that do not match its version. There is, however, a configuration option to disable this safety feature.