The Posuk says (Shemot 25:8):
וְעָשׂוּ לִי מִקְדָּשׁ וְשָׁכַנְתִּי בְּתוֹכָם
And let them make Me a sanctuary ('Mikdash'), that I may dwell among them.
I have two questions: first, this was the instructions for the Mishkan not the Beit HaMikdash; second, what's בְּתוֹכָם "among them"? Why the plural? Plain translation would indicate we are speaking of the Mishkan.
“Mikdash” is a more generic term for a “holy place” or (following Rav S. R. Hirsch) a “sorce of holiness”. The Mishkan was the specific Mikdash built in the desert. (Note the same root SH-K-N in mishkan and v’shakhanti.)
For your second question, God is not promising merely to dwell in the tent built for Him but to be an active Presence within the nation: “Let them build Me a holy place and I will dwell among them.”
The Gemara (Eruvin 2A-3B) actually uses this verse to prove that a Mikdash is sometimes called a Mishkan. (Actually, that Gemara actually says that "Mikdash" and "Mishkan" are interchangeable terms).
The Lubavitcher Rebbe explains that according to Rashi (the simple explanation of the text), one cannot say that "Veshanchanti Besocham" is a result of "Veasu Li Mikdash" (like some commentaries explain that through building the Mishkan, Hashem will dwell in each Jew or that through dwelling in the Mishkan it's considered that Hashem dwells in each Jew) since then the verse should have stressed "Vehachanti Bo" (the main dwelling is in the Mishkan) and then said in a different location how that translates to Hashem dwelling among the Jews.
Therefore, according to Rashi, the verse should be read "And you shall make me a tabernacle (and as a side point, through keeping the command to build the Mishkan, I will I will dwell among you) according to all that I show thee, the pattern of the tabernacle, and the pattern of all the furniture thereof, even so shall ye make it."
