Hi, well for starters, 'diffused' light is not what it needs, but full sun is - OUTside. Junipers (no matter what the tag or clerk might have said) can't live indoors for very long and must be outdoors for life. You didn't say where you live, however, and it's important to know before you do anything right now as there are other factors that are very important. Don't water it again until you get back here (it's ok for a lot of the soil to dry out, much better than being too wet).

I live in Virginia, near Richmond.

Well it's definitely warm enough there so your tree can live outside for life, but for now, as it's been inside for a while, it might be best to keep it in until the last frost of the season has passed in your area, and then put it outside. It's important though that it be planted in the right mix because rain could easily do it in if not, and store-bought junipers are usually planted in cheap potting soil with little grit for drainage. What you need is some help to do that right, and learn a lot more about growing your tree (and future ones). Go to http://freeplone2.openia.com/abs/club-listings/state-listings/virginia for bonsai clubs nearby to join - it's the best way to learn, because people in your area who know the environment, local nurseries, and who can show you just how to do things are a great place to start.