Plasmid in the nucleus and gene expression
If you transfect cells with plasmid then these plasmids need to go into the nucleus (otherwise they wouldn't be transcribed). Getting the plasmid either happens during cell division (when the nucleus is not present) or by adding a signal sequence to the plasmid which induces the import into the nucleus through nuclear pores. The SV40 sequence is an example of such a sequence. 

If you have the right promoter present on the plasmid, you can basically express any protein in the cell (unless it's toxic). The promoter either needs to come from the organism from which the cells are or has to be something universally activated like the CMV promoter (from the cytomegalo virus).

A good example for this is the expression of green fluorescent protein (GFP) which originates from jellyfish and can be expressed in cells to make them glow green (from here):



Untransfected cells or cells transfected with an empty plasmid (without GFP) don't glow.

Targeting specific cells or control the specific activation of genes only in one cell type is possible. For both you need sequence (and cell specific) promoter sequences which either allow the nuclear import (see reference 1-3) or the expression from a cell type specific promoter (see reference 4).

This has been done in the liver of mice in which the ApoE has been knocked-out. The expression of the human ApoE gene with a specific promoter led to the correction of the hypercholesterolemia the mice were suffering from. This needs a lot of knowledge on the promoter as well as on the other transcriptional regulation elements in the specific gene (see reference 4).

References:


Cell-specific nuclear import of plasmid DNA.
Tissue-specific and transcription factor-mediated nuclear entry of
DNA.
Progress and prospects: nuclear import of nonviral vectors.
Sustained liver-specific transgene expression from the albumin
promoter in mice following hydrodynamic plasmid DNA delivery.