Hi, to be able to use your BeoLab 3500 as a Powerlink speaker, you need ..patience, and some extra tools!
There is a French workshop called Madsystemes that makes adaptors to connect Powerlink on one end and Masterlink on the other, google the name and you should find their website, it costs around EUR 200.
Another solution which I use with my BeoLab 2000 is to connect the Powerlink or audio link out of your BeoSound (or your Ipod/PC via a jack to DIN 5 adaptor) to a MCL Converter (type 1611) on one end, and then go to the Lab 3500 via Masterlink, however if you want to control the volume from your audio source, you'll have to use both Powerlink and aux link to feed the MCL converter. I use only the aux link out, not the Powerlink, which gives me independent volume control on the Lab 2000. In that case, you'll have to press "TV" on a B&O remote to activate the Link speaker, otherwise it won't turn on.
Not very straightforward, but it works!
Reunion Island is greeting you!