Hi Ben321,
Unlikely that the makers of Nasm would include all possible files required, since Nasm will compile in a lot of places besides Linux - let alone Damn (too?) Small Linux. I think libc.so.6 is a standard C library, but I think it's actually a link to some other file. I suspect what you have to do is make a link to some other file - but I don't know which one. Maybe ask DSL? In my opinion, they're the ones that ought to be providing the files you need!
Googling for libc.so.6, I come up with a lot of references to "RPM". Is this an "RPM thing"? I've never had any luck with "RPM" - I consider it a "redhat thing" (everybody wants to be "one ring to rule them all"!) I just download the source and do "configure" and "make install". Try that, if you haven't. Wish I could help you more.
Best,
Frank