You cannot regerate a true mixed bed deionizing resin, unless you somehdow seperate the resins:
Reverse Osmosis/Deionization Systems to Purify Tap Water for Reef Aquaria
http://reefkeeping.com/issues/2005-05/rhf/index.htm
Recharging DI Resins
When DI resins are present as mixed bed filters, they are essentially one-time use devices that must be thrown away when depleted. These are the types supplied with many commercial RO/DI systems, and are what I use. Separate bed DI resins have certain advantages, however. In particular, if they are kept in different cartridges, they can be recharged.
The recharging process is essentially the reverse of the deionizing process (Figures 6-8). A strong acid (usually hydrochloric acid, HCl) is used to swap H+ for all of the positively charged ions on the cation-binding resin (Na+, K+, Ca++, Mg++, etc.). Likewise, a strong base (usually sodium hydroxide, NaOH) is used to swap OH- for all of the negatively charged ions on the anion binding resin. Both hydrochloric acid and sodium hydroxide are readily available and inexpensive. They are, however, potentially dangerous to work with. The detailed procedures and safety precautions are beyond the scope of this article, but can be found elsewhere online.
the link from it goes to:
http://www.reefs.org/library/article/g_deutschmann_instruct.html