After investigating this issue further, the non-functioning mouse issue appears to be a compatibility problem with some unsupported graphics cards (mainly SiS and Intel integrated versions).
These cards are unsupported by Dungeon Runners so playing the game with these cards is at your own risk, however playing in windowed mode does appear to correct this issue for many folks with this problem.
The game may run when using an unsupported video card, but Dungeon Runners is not tested or maintained to work with these cards, so issues may arise during play.
Please see the stickyed post at the beginning of this forum for the Minumum hardware specs for Dungeon Runners which includes supported video cards.
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~Ripper