I wasn't able to drink 6 cups for the first few days either. (I'm on Day 9). But I worked my way up to it-- by day 5 I was drinking 6 cups without difficulty.
I would really discourage using liquid magnesium citrate during the cleanse. The book directs taking herbal laxative tea (such as Smooth Move) and Seasalt water flushes to aid the body in eliminating. The tea works fine for me.
(BTW-- I have taken liquid magnesium citrate several times prior to various diagnostic tests [I have cystinuria, a kidney condition]. It's always resulted in severe cramping and nausea for several hours for me, and leaves me weak and severely dehydrated.)