Don't know what's wrong with you, I copy-paste at xterm(1) frequently. There are three methods which work for me.
Method 1: use your mouse to select text using left and right mouse buttons, then press middle mouse button where you want to paste the text (if you don't have middle mouse button, then X.org emulates its click when you press left and right mouse buttons at the same time). Method 2: again use your mouse to select text, but this time we'll place it in different copy buffer using Ctrl + Insert combination. Then to paste the text you just have to press Shift + Insert combination. This one is like Ctrl + C and Ctrl + V in Microsoft Windows. Method 3: learn general tmux(1) commands by reading its manual, you need to enter copy mode: Ctrl + B [ By default it uses Emacs-like key bindings, so mark the start of the text you're interested in: Ctrl + Space Then by using cursor commands select text and press Alt + W to copy it. In the end in the same tmux session (at any pane and any windows) press Ctrl + B ] to paste yanked text.
