It depends what interface you are using. If you used Peakd and subscribe to the community there will be a drop down box at the top of the text editor and you can select the desired community from there. Or just navigate to the community and click new post. Tagging it doesn't put it in the community, I think unless you use it as the first tag, but that's the long way around and I'm not sure if it works still. The other two methods are best.
The image below shows the drop down box which is above the text input box and will display all the communities I subscribe to. I simply select the one from there and that's it.
Below shows it selected.
There's the other option...Navigate to the community and click the blue box.
Sounds workable I'll try option number two right now.
:)>