We are no longer supporting the integration which is why you cannot find it in Admin. I'm not sure about your other questions. I'd have to dig deeper and we will definitely get this straight with their sales team.
We were in the same spot- Paid for a data.com connector license and then they dropped the APIs needed to integrate with Marketo just a few months later (It dropped in Sept 2013 with the "upgrade" to data.com connect).
There might be some way to customer engineer the API calls that they do permit; and do something with webhooks, but it really looked far too complicated.
We ended up signing a contract with InsideView to do the exact same thing. InsideView does it via webhooks: Its very easy to use; bascially just setup the webhook connector once, go into their platform and configure the field mapping, then add a "call webhook" step from within marketo wheneever you need a lead enriched. If you wanted all incoming leads enriched, you could simply make a simple program, with "lead is created" trigger, and flow be "call webhook" and that would do the trick.
InsideView's data is much better than data.com for the quality of company data, but the lead data completeness is about the same. Very happy with the service so far; send me a note at email@example.com if you want the sales person I worked with at InsideView or if you've got any more questions.
Naor- Our data.com connect pricing used to be $90 a month for 300 enrichments per month, and then we would usually purchase an addition 10,000 credits for the entire year for another $4,000 or so.
With insideview, our pricing is around $350 per month and that covers up to 500 enrichements. Its definelty more pricey than what we used to pay (annual sum is InsideView: $4200 for 6,000 lead enrichments, Data.com connect used to be around $5,000 for 13,600 lead enrichments)
Definetly a little more pricey, but its easy to use and I couldn't find anything else at the time that would do the same thing.