P-values in Minitab can be found using the Command Line Editor. If I want to find a single p-value for a 2^3 experiment with 3 reps (that's 16 degrees of freedom), I would type in the Edit>Command Line Editor window:
cdf 1.19;
t 16.
This actually gives me the area to the left of 1.19. You should subtract this answer from 1 and then double it to get the p-value.
If you want a critical value for the same situation, you would type in the Edit>Command Line Editor window:
invcdf .975;
t 16.
Any effect's T statistic whose absolute value was larger than the answer provided by Minitab would be significant. If you want to find p-values for several effects simultaneously, enter their T statistics into a column (e.g., C1) and enter in the Edit>Command Line Editor window:
let c1=abs(c1)
cdf c1 c2;
t 16.
let c3=2*(1-c2)
That last step computes p-values for you.
You can also use the pull-down menus for these computations. To repeat the cdf calculation, you could pull down Calc>Probability Distributions>t. Click Cumulative Probability, enter 16 in the Degrees of freedom dialog box, click Input Constant, and enter 1.19. The cumulative probability will show up in the Session window. It should then be subtracted from 1 and doubled to obtain the p-value.
To find the critical value, again pull down Calc>Probabilty Distributions>t. Click Inverse Cumulative Probability, enter 16 in the Degrees of freedom dialog box, click Input Constant, enter .975, and the critical value will appear in the Session Window.
For a column of p-values, you would indicate the column under Input Column, rather than Input Constant.