Download xchat2. At least 2.0.8 is needed because xotp uses the new Perl interface.
First open xotp.pl and edit the use lib line to match the location of IMOTP.pm.
Now open IMOTP.pm. @pads should list all of the involved one-time pads. @paths is the search path; where the pads can be found. Last comes the %nick2pad associations, where IRC nicknames are mapped to padnames. The nick to padname associations must be conistant between users. The first pad is a send pad, second is the receive pad. User A's send pad should be user B's receive pad, etc. Example:
$nick2pad{"user1"} = ["send_pad", "recv_pad"];
The user can also be a channel; in which case the recv_pad is ignored, and the send pad should be set to the pad used for sending to the channel.
There are a few more settings that can optionally be changed. $nohigh strips the high bit. $asciilc lowercases all outgoing messages. $show_enc is analogous to "Replace AIM window" in otpaim--it will not show encrypted text, either being received or sent. $align_on is the multiple of bytes on which ciphertext should be aligned. Higher reveals less information but uses more pad.
Start xchat2, go to X-Chat -> Load Plugin or Script, and load xotp.pl. If you want it to be loaded on startup, place xotp.pl in ~/.xchat2. Make sure it can still find IMOTP.pm.
Open an OTP encryption tab with /otp nick (this is similar to /query nick). There should be an *OTP* tab where you can type and receive encrypted messages. Enjoy.