Many, but not all MQ indicators are available via the MultiCharts trading platform. The indicator listings in the store are generally accurate in identifying which indicators have been ported to MultiCharts. If an indicator is available in MultiCharts, then you'll be able to download it via the Indicators download page on the BCT website. If the option to download the MultiCharts version isn't available for an indicator, then it hasn't yet been ported to MultiCharts. In that case, contact BCT support to let them know that you're interested in having the indicator made available in MultiCharts. In most cases, when you buy an indicator from the Base Camp Trading site, you can use the indicator on all platforms that it's currently supported. So for example, if you buy an indicator to use on NinjaTrader, you can usually also use it on MultiCharts if there's a MultiCharts version that's available. Again, if you are provided with an option to download it, then you should be able to register and use it.
A note to indicator developers: Yes, MultiCharts uses a language that's almost identical to TradeStation's Easy Language, but it's often the case that the complex indicators we create aren't able to be created by simply using the Easy Language code. It can take time to come up with a way to achieve some of the things that work differently in TradeStation, and not everything that TradeStation does can be done in MultiCharts (and perhaps vice-versa).