Did the Lubavitcher Rebbe (Rav Menachem Mendel Schneerson) say anything about wearing Techelet made from the Murex trunculus (as claimed by Ptil Tekhelet to be the correct Halachic Tekhelet) nowadays?
If so, where and what did he say?
Actually, the Rebbe, Rabbi Menachem Mendel Schneerson, did give a response which can be found in his Igrot Kodesh when he was asked in the 1960's (might have been early 1970's). He said that he had nothing to add to the response of the Rebbe Rashab, Rabbi Shalom Dovber Schneersohn, on the question.
The Rashab was asked specifically about the Radziner techelet which was from the cuttlefish. The Rashab said that as much as he wanted to be able to say the Radziner techelet was legitimate, he could not after reading everything the Radziner Rebbe had provided him about it. This letter can be found in the Igrot Kodesh of the Rashab.
The Ari z"l does say that the the 'shefa' that is drawn into the world through the mitzvah of techelet would not be possible until the coming of Moshiach. For those interested in reading the details of this, it can be found in Sefer Divrei Shalom - Minhagei Rashash, volume 1, by Rabbi Avner Afgin.
The Rebbe did not discuss the current techelet from the Murex Trunculus, also called Heptaplex Trunculus.
From the perspective of the teachings of the Rebbe, that we have crossed the threshold from the 'Days of Galut' to the 'Days of Moshiach' would at least offer the possibility that the requisite mentioned in the name of the Ari z"l has been fulfilled. This is one of the reasons why some Lubavitchers wear the techelet from the Murex Trunculus today.
I didn't find a reference to techelet in the online archive of the the Lubavitcher Rebbe writings but there are two references to Rabbi Sholom DovBer, the fifth Rebbe of Lubavitch, and the Arizal both writing that we will only know about the chilazon (who produces techelet) when Mashiah comes.
This must be why Chabad rabbis do not wear techelet.