At what point did he "admit to cheating", which seems to be what they are trying to convey?

Well, he never really admitted to gaming campaign financing, did he?

The Washington Post: Wp dyn Content Article

Sen. Barack Obama's presidential campaign is allowing donors to use largely untraceable prepaid credit cards that could potentially be used to evade limits on how much an individual is legally allowed to give or to mask a contributor's identity, campaign officials confirmed.

Let's expand on this, and say what WaPoo didn't: Obama turned off the AVS (address verification service) feature for his credit card donations. Know what that did?

It meant there was no way to trace how much foreign cash he got. Oh, that must have been an oversight!, one might try as a rejoinder. Nope. You actually pay MORE to the credit card companies when you do that, so that was clearly done deliberately.

Yes. Obama knows all about cheating.

The Obama campaign has shattered presidential fundraising records, in part by capitalizing on the ease of online giving. Of the $150 million the senator from Illinois raised in September, nearly $100 million came in over the Internet.

Not a smidge of corruption!
Our resident fact-checker will come along and try to cite this as evidence that Obama fixed the glitch:

politico.com - Obama campaign fined

...by trying to claim that a thorough investigation was done, yadda yadda yadda.
'At the time, the 2008 campaign was record breaking, with over 3 million grass-roots donors,' Obama campaign spokewoman Katie Hogan said. 'The very few outstanding questions about the $750 million that was raised have now all been resolved.'

$375k to enable a flood of untraceable foreign cash? There's a deal that any shady politician would take. And they did.