There's no particular reason you need to get an xterm, if you're satisfied with the Mac native "Terminal" program. In fact, things like cut-and-paste using command-c and command-v don't work in xterm, so it can be kind of frustrating.
Still, if you really need to run an xterm, you can launch one from a regular terminal window. Just type in 'xterm'. If you get a "Command not found" error, type in '/usr/X11R6/bin/xterm'
If you get an error like "xterm Xt error: Can't open display: " then type in something like this "setenv DISPLAY localhost:0" and then enter the xterm command again.
PS you can copy/paste in X11 by selecting the block you want to copy and then option-clicking. The selected text wil be pasted where the cursor is. Actually, I just experimented and option-click in X11 will paste the clipboard cut or copied from native apps like Terminal and Safari. I don't know how you can put something in X11 on the clipboard to export to native apps though.