I know this subject has come up quite a few times so far (with different distro's of texlive). See for instance:
Missing tlmgr texlive 2012
tlmgr: TeX Live 2011 is frozen forever
Texlive installation is missing tlmgr, how to fix?
http://the-praise-of-insects.blogspot.com/2011/08/tlmgr-not-available-for-ubuntu.html
http://forums.fedoraforum.org/showthread.php?t=260451
None of them provided a solution that worked for me. The last link implies that the Ubuntu developers decided not to include tlmgr in the texlive distribution.
I installed texlive afresh via the following instructions: https://askubuntu.com/questions/160676/problems-installing-tex-live-2012-on-12-04
My texlive distro apparently lives here: $ whereis texlive: /usr/share/texlive
Any clue where to find tlmgr or how to activate it without reinstalling the texlive distribution all over again?
Best Answer
As both Debian maintainer of TeX Live and author of tlmgr, I would like to state my personal ideas about it:
First, with the Debian native packages of TeX Live there will be in foreseeable time no tlmgr available. Even if it will be available, then only for managing TEXMFHOME or TEXMFLOCAL (for the sysadmin) (by using the user mode patches for tlmgr which are badly outdated at the moment).
On the other hand, it might not be necessary to have tlmgr available. Package installation, removal and configuration is done by the APT family (apt-get, aptitude and respective GUIs). Package search can be done with apt-cache search. That leads to the following suggestions:
For normal users, that are users who are not developers of (La)TeX/Font/etc packages, nor play with the latest font developments (esp. otf math fonts), for these users it is better on Debian/Ubuntu to stay with the native packages instead of installing your own copy. Especially if one does not know exactly about basic sysadmin tasks (PATH search priority, permissions etc), since if you don't know it is bound to create problems.
If you are developer, or have experience enough with both basic sysadmin tasks, using TeX Live upstream together with either a proper equivs package for texlive, or (disk space allowing) double installation.
Furthermore, if you are using Debian testing or sid (and if it exists something similar in Ubuntu), then after release of wheezy I plan to do monthly updates of the texlive packages based on the current tlnet status. Thus even Debian people would come - with a short delay - to latest packages.