Huehue custom dance pads for DDR, we built a set for my sisters birthday a year ago. though they were basic, they had EL panels as the light source for each of the arrows, however no uController was needed, just used pressure switches same as the existing ones in a dance pad (we gutted some red octane pads for their pressure switches and the connection to go to the PS2) and jsut had them trigger the EL panels under the arrow decal that got put on some polycarbonate.
For you though, so im guessing what you want to know, how to pick up the signal when the person is stepping on an arrow. and then tell the LEDs to turn on in the manner you want (you mentioned that on stepping on it, the arrow will flash, and then if your weight is kept on it it’ll continue flashing until you release). Well if its basic and you just want them to light up. all youll need is an interrupt for each of the arrows, that upon being pressed, then run a code. Something like, turn on LEDS, wait .25 secs, turn off, wait .25 secs, turn on. and just have it repeat that while the button is pressed. Electrically, well one I wodner why you choose LEDs, how are you going to mount them in the dance pad, are you buildign a sturdy pad, or jsut taking a mat, or foam insert pad and thinking of adding the lights because LEDs won’t like being stood on repeatedly, or jumped on for that matter (hence why my project opted for EL panels, they seemed like they could take a better beating).
But like what pylon said, 100 RGB LEDs (read, 300 LEDs actually) is going to be a significant amount of power. Youll need several PWM drivers at least to drive the RGBs,
http://www.sparkfun.com/products/10136 seem pretty cheap, each can control 16 channels (so youd need 2 for each color for each pad, and then 3 pair so 6. you could have the extra pins go over and control a different color, or the lights on a different arrow, but then coding becomes more difficult.) And youll also need a power source external to your arduino and any ICs just to run the LEDs.
also i doubt any of the LEDs your going to be buying are going to be 12 or 24v. everything going to want 5. Also whats the amperage rating on that power supply. like pylon said, about 10A of power will be needed for all the LEDs on, thats a pretty significant power supply.