For your grid question, let's assume the name of your question is grid1 and we want to select the first row and first column (grid1_r1_c1). Here is how you would use this function. Place the following code into the footer of your grid1 question (we place this code in the footer because we want it to be executed after the entire question has been written to the screen).
The code is almost identical for your select question. Let's assume the name of your question is select1 and you want to pre-select the first member (select1_1) of your predefined list. Here is what the code would look like:
If you have multiple check-boxes or radio buttons you want to pre-select in a question, you can copy and paste as many SSI_SetSelect lines as you need. Just keep an eye on those pesky semi-colons at the end of each line.