Unfortunately, I'm pretty sure LinkedIn shut down their Lead Accelerator product.
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This is only somewhat true. LinkedIn transitioned Lead Accelerator into their self serve Campaign Manager.
However, the core Lead Accelerator functionality still exists, just with a different name: LinkedIn Account Targeting.
You can follow these instructions to connect the new LinkedIn Account Targeting just as you would Lead Accelerator, with a few notable differences. Hopefully, Marketo will update their nomenclature on this soon enough.
And to answer Stephen Jeong's query, yes, you should be able to use Launchpoint to create a LinkedIn connection to multiple LinkedIn accounts, and/or to multiple Marketo instances.