1-n+1/2=position of the median
n=number of values or the sample size
2-you then need to add a cumlative frequency column to the table and the number you got for 1- needs to be fitted in. The group it is in is the median group, but i'm not sure how you work out the actual value.
for example-
weight frequency cumlative frequency
1<n<2 4 4
2<n<3 10 14
n+1/2=14+1/2=7.5th number
7.5 is larger than 4, but less than 14, so is in the 2<n<3 group
2<n<3 is the median group.