mEq is a unit used w/electrolytes - basically how many ions of the electrolyte are present
I've seen it used in IV's (Na+, Li+, and K+ ions). Usually they give you the concentration in mEq/liter. Today the units are usually ug/liter.
To convert mg to mEq one needs to know the form the potassium is in. Is it a compound or ion, etc.
for KCl - usually in IVs
1 mEq of KCl = 1/1000 x 74.5 g = 0.0745 g = 74.5 mg
for K+ ions
1 mEq of K+ = 1/1000 x 39.g = 0.039 g = 39. mg
Note: this is because K+ has a valence of 1
I hope this helps to steer you in the right direction.