You need your SAP Service Marketplace username and password.

1. Visit http://service.sap.com/
2. Under the "SAP Support Portal", there is a link titled "Software Download". Click on it.
3. In the page that opens up, the left pane is a tree / hierarchical view. Follow this path: SAP Software Distribution Center --> Download --> Support Packages and Patches --> Entry by Application Group. Click on it.
4. In the right pane, a few links will come up. Click on "Additional Components".
5. In the right pane, a few more links are displayed. Choose "SAP RFC SDK UNICODE". (You always need the Unicode libraries, irrespective of whether your SAP Server is unicode or not).
6. You can download either version 6.40 or 7.00 or 7.10
7. After choosing your platform, a list of files available for download will be displayed (usually just one, since you've narrowed it down to version number + platform). Add it to your download basket.
8. A window will pop up confirming that it has been added to your basket. Close it for now.
9. Go back to the page where the Additional components were displayed. (Step 5). A few links above "SAP RFC SDK UNICODE", you should see "SAPCAR". Click on it.
10. Choose the latest version of SAPCAR available, and the one appropriate for your platform (32/64 bit, Windows/Linux/etc).
11. After choosing your platform, a list of files available for download will be displayed (usually just one, since you've narrowed it down to version number + platform). Add it to your download basket.
12. A window will pop up confirming that it has been added to your basket. In that window, there is a link that says "Download Basket". Click on it. Another page will come up with a list of all items in your basket, and a link which says "Get Download Manager". Click it. The Download Manager is a desktop application which you henceforth need to use to download stuff from the SAP Marketplace - the online interface (steps 1 to 11 above) is just a way to add things to your download basket.
13. Once the download for the Download Manager completes, install it. Start it, you'll be prompted to enter your SAP Service Marketplace username/password. Once done, you should see a list of all files in your basket which you can download. Choose the ones you want, and download.

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.