I could be wrong but ad-hoc is not what you want, you want infrastructure mode. The pc needs to act as the access point for the Roku.
Another alternative might be to use a mac address spoofing utility on the PC to fake the mac address of the roku to the access point. authenticate to the wifi on the pc, then turn off the mac spoof, turn on the roku.
If your apartment has wifi though, you will be competing to use your device with everyone else on wife. this might mean that either A your neigbors don't have a good time on the internet, or B you don't get to watch Roku in any kind of decent quality, or C everyone suffers.
Best if you can get your own telephone line and DSL or cable internet unless your apt building has a really fat pipeline.
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