How can I create RAR archives on a Microsoft Surface RT tablet that has Windows RT? I can only create ZIP archives currently.
You can't, for now. Since it's a proprietary closed format and nothing except RAR or WinRAR can create RAR archives, until an official Windows Store app is released this is not possible. (Well, it might be possible if someone reverse-engineered the format, but that would be illegal and I highly doubt such an app will ever be approved by Microsoft or survive a legal challenge.)
Since the UnRAR code is open source, programs like 8 Zip can extract RAR archives (using 7-Zip's DLL/code for archive support). An alternative to the common ZIP format though is 7-Zip's 7z, which is on par in terms of compression ratios with RAR, and being open source is supported completely (i.e. both creation and extraction) by 8 Zip.