You need your SAP Service Marketplace username and password.
At this point, I'm assuming your download went smoothly, and you have 2 files downloaded - SAPCAR.exe, and <somenumber>.SAR. SAPCAR.exe is SAP's uncompressing/unarchiving tool. The .SAR file you downloaded is the RFC SDK Unicode library. To unarchive it, run this command:
SAPCAR -xvf <.SAR filename>
You should now see folders named "include", "lib", "bin", etc. In the "bin" folder, there are 6 dlls present. Copy them to your System32 / SysWow64 folder as appropriate (depdending on whether you are installing the 32/64 bit Adapter Pack, you're on a 32/64 bit machine, you're using Native64/WOW64, etc).
Then, read this post to verify whether the RFC SDK Unicode Libraries can be successfully loaded by a client application.