To use the chart, simply compare the psionic ability listed for the creature. Then, based on the chart (and possibly influenced by the creature's HD), assign it a psionic level and PSPs. Since the combat modes are named the same as the early editions, the alphabetic indications are easy enough to interpret...
|Attack Modes||Defense Modes|
|A. Psionic Blast||F. Mind Blank|
|B. Mind Thrust||G. Thought Shield|
|C. Ego Whip||H. Mental Barrier|
|D. Id Insinuation||I. Intellect Fortress|
|E. Psychic Crush||J. Tower of Iron Will|
For specific disciplines possessed by the creature, all of the classic disciplines are already included (and, more importantly, are named with the original names, rather than some of the newer edition names). If there is a specific level of mastery noted for the discipline, consider that the psionic level. Otherwise, defer to the psionic level as determined by the Psionic Ability conversion above. Also, there's really no problem with making all of the creature's disciplines operate at creature's psionic level, regardless of the indications in an original source.
If you're looking to convert something "newer" than AD&D (say, a creature native to Athas from 2e), and the ability is something a little out of the ordinary and not included, this is easy enough. All of the 2e abilities are already noted as a major science or minor devotion, and assigned as part of a discipline group (e.g., clairsentient, psychometabolic, etc.). That means the PSP costs are already prescribed (based on those factors) by the Basic Psionics Handbook. You just have to make sure the duration of the ability is appropriate for BX/LL (eliminating the maintenance costs), and tweak to eliminate the need of power checks. For the former and latter, I'd suggest finding a similar ability in the Basic Psionics Handbook (or possibly something from the psionics SRD) and using that as a guide (seeing as there are 114 abilities included in the BPH, it's almost guaranteed there's something similar to what you may be looking for).
And did I mention this was coming soon...