If you simply specify a number of charges they will auotmatically be arranged across the field. See the discussion on arrangements for more information.
Charges may be placed in a specific location by prefixing or appending the charge descriptions with one of the following:
Central locations (from top to bottom) In Middle chief, in base, in honour point, in fess point, in nombril, in flank, in abyss
Side locations - in dexter/sinister chief, in dexter/sinister flank, in dexter/sinister base
In quarters - in first/second/third/fourth quarter
You can place charges on an ordinary. In most cases he space available to do this is quite restricted some only a small number of charges is allowed, an error message will be generated if there isn't room to place all of the charges. Similarly, some numbers of charges don't really make sense on some ordinaries, again an error message will be shown if this is the case.
You can also place charges in relation to an ordinary by using the contsruction ordinary between number charges, or number charges within ordinary. As usual this only makes sense for certain ordinaries with particular numbers of charges.
Note that in chief is an arrangement, NOT a location (compare with in pale, in fess). What you probably mean is in middle chief.