Hi Pengcheng Yang,
There is currently no way to design primers from within IGB. My advice would be to use IGB to identify regions of alternative splicing, and then use primer 3 to pick primers.
1) Identify alternative splicing event of interest in IGB.
2) Copy the DNA sequence directly from IGB and paste it in primer 3 (http://biotools.umassmed.edu/bioapps/primer3_www.cgi).
3) Use primer 3 to pick primers.
4) Visualize the primers in IGB using the Advanced Search tab to verify that they capture the alternative splicing event.
We're in the process of greatly expanding support for user developed plug-ins in IGB. The page on primer design in the IGB user's guide is a placeholder at the moment, but is a feature we would like to add in the future.
Nowlan