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 `depmod` and `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.