Strange. Please tell me what I’m doing wrong. I just wanted to try out the log/exp interpolations. Start playing an instrument at line 3, set volume to c0. Jump to line 23 and set volume to 20. Now, draw a selection from line 3 to line 23 (inclusive). Click on either of the three interpolation buttons. Nothing happens here but I feel awfully stupid since I must be missing something. (last time I tracked was about 10 years ago so please cut me some slack) 
Try pressing CTRL+I instead.
edit:
Ah, sorry, I know what you meant now. I just quickly glanced at your post at first.
The Lin/Log/Exp options work here for me. Have you made sure that “Select Section & Content Mask” is set to “Selection” before hitting the interpolation buttons?
Also, you said “c0” in your post. That’s not actually a valid volume setting. The range is 00 - 40, so make sure that you’re using values within that range.
Ah, thanks! My range was off, using volumes 00 and 40 interpolates perfectly. 
Thanks for the lightning-fast answer!